The OP, Mahaus, is wrong about Titan. Titan is unsuited for being habitable for humans because it has an excape velocity which is too low to retain gases such as oxygen, not because of its lack of a magnetosphere t o prevent the solar wind fron knocking molecules out of its atmosphere.
The first necessity to maintain an atmosphere on a world is sufficient escape velocity. Having a strong magnetosphere to defect particles of the salor wind away from the world and its atmosphere is a secondary consideration.
Note that the planet Venus has a highly dense atmosphere, despite having a very weak magnetossphere compared to Earth.
In 1967, Venera 4 found Venus' magnetic field to be much weaker than that of Earth. This magnetic field is induced by an interaction between the ionosphere and the solar wind, rather than by an internal dynamo as in the Earth's core. Venus' small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation.
The weak magnetosphere around Venus means that the solar wind is interacting directly with its outer atmosphere. Here, ions of hydrogen and oxygen are being created by the dissociation of neutral molecules from ultraviolet radiation. The solar wind then supplies energy that gives some of these ions sufficient velocity to escape Venus' gravity field. This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, whereas higher-mass molecules, such as carbon dioxide, are more likely to be retained. Atmospheric erosion by the solar wind probably led to the loss of most of Venus' water during the first billion years after it formed. The erosion has increased the ratio of higher-mass deuterium to lower-mass hydrogen in the atmosphere 100 times compared to the rest of the solar system.
Vwnus has lost a lot of atoms of lighter elements from its atmosphere due to a week magnetosphere.
But Venus still retains a very dense and massive atmosphere:
Venus has an extremely dense atmosphere composed of 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases including sulfur dioxide. The mass of its atmosphere is 93 times that of Earth's, whereas the pressure at its surface is about 92 times that at Earth's—a pressure equivalent to that at a depth of nearly 1 km (5⁄8 mi) under Earth's oceans. The density at the surface is 65 kg/m3, 6.5% that of water or 50 times as dense as Earth's atmosphere at 293 K (20 °C; 68 °F) at sea level. The CO2-rich atmosphere generates the strongest greenhouse effect in the Solar System, creating surface temperatures of at least 735 K (462 °C; 864 °F). This makes Venus' surface hotter than Mercury's, which has a minimum surface temperature of 53 K (−220 °C; −364 °F) and maximum surface temperature of 700 K (427 °C; 801 °F), even though Venus is nearly twice Mercury's distance from the Sun and thus receives only 25% of Mercury's solar irradiance. This temperature is higher than that used for sterilization.
Venus is obviously not going to lose its atmosphere to space anytime soon. It has kept an atmosphere many times as dense as Earth's for billions of years.
The less massive an astronomical object is, the lower its escept e velocity is likely to be. And the lower the escape velocity, the faster the object looses whatever atmosphere it has. And the lower the escape velocity, and the stronger the solar wind hitting the atmosphere is, the faster the solar wind will accelerate ions to the lower escape velocity.
So on a low mass world with a low escape velocity, the weaker the magnetosphere is the faster the solar wind will accelerate the loss of atmsophere. A weak magnetosphere is most effective in removing atmosphere from a world that has so low an escape velocity that it is losing atmosphere anyway. A weak magnetosphere makes a bad situation worse.
And as a general rule, the more massive a world is, the stronger its magnetosphere is likely to be.
Someone interested in writing about habitable planets, moons and other worlds should
read Habitable planets for Man Stephen H. Dole, 1964, 2007.
It includes scientific discussions of many habitability related factors including the possible mass range of a habitable planet.
Note that your example of a low mass world, Titan, has a mass of 0.0225 Earth, while Dole's calculated minimum mass for a planet to retain a dense oxygen rich atmosphere is 0.195 Earth, 8.6666 times as massive as Titan. So that explains why Titan is basically airless.
Actually, of course, Titan has a significant atmosphere, with a surface pressure greater than the surface pressure of Earth's atmosphere. Like Earth's atmosphere, Titan's atmosphere is mostly nitrogen, but unlike Earth's atmosphere, Titan's contains no free oxygen.
One major reason why Titan has such a dense atmosphere billions of years after forming is that Titan orbits Saturn, which orbits the sun at a distance of 9.5 Astronomical units, which is 13.194 times the distance of Venus from the Sun and 9.5 times the distance of Earth from the Sun. So at Titan's distance from the Sun, it receives only 0.0110 times as much solar radiation as Earth, and only 0.005744 times as much solar radiation as Venus.
That means that the average temperatures in the upper layers of Titan's atmosphere are much lower than the average temperatures in the upper layers of Earth's atmosphere. So atoms move much slower in the upper layers of Titan's atmosphere, the layers that lose atmosphere, than they are in the upper layers of Earth's atmopshere. This enables the lower escape velocity of Titan to retain atmsphere much longer than it would if Titan had Earth's temperature.
I also note that if Titan receives only 0.0110 as much radiation from the Sun as Earth and only 0.005744 as much a Venus, that includes the solar wind. The solar wind would obvious take a much longer time to knock away Titan's atmosphere at the distance of Saturn than it would take at the distances of Earth or Venus.
Anyone interested in the possibility of habitable exomoons orbiting giant exoplanets in other star systems should read:
Heller, Rene, and Barnes, Roy "Exomoon habitability Constrained by Illumination and Tidal Heating" 2013.
Heller, René (September 2013). "Magnetic shielding of exomoons beyond the circumplanetary habitable edge". The Astrophysical Journal Letters. 776 (2): L33.
Acccording to the later paper, exomoons orbitating larger giant planets at distances between 5 and 20 planetary radii will be within the planetary magnetosphere.
Saturn has an equatorial radius of 62.268 kilometers or 36,184 miles, so moons orbiting Saturn at distances of 311,340 to 1,245,360 kilometers should be within the planetary magnetosphere. Tital orbits Saturn at a distance of 1,221,630 kilometers and so may have been protected from losing atmosphere to the solar wind by Saturn's magnetosphere.
In any case, Titan does have a dense atmosphere, despite it's low mass and escape velocity,perhaps being able to produce or otherwise acquire atmosphere faster than it is losing it.
Of course if the story involves a low gravity world with a dense atmosphere which is breathable to humans at the surface and has a temperature suitable for humans at the surface, there is a problem. Titan's doesn't satisfy either requirement, and probably would not be able to retain its atmosphere if it was at Earth's distance from the Sun.
What is needed is a world with the surface gravity and escape velocity of Titan, and with temperatures at the surface similar to those of Earth, but almost as cold as those of Titan at the outer layers of it's atomsphere where atoms escape into space, and with a breathable atmosphere at the surface.
One way to do so might be the make the world an exomoon orbiting a giant exoplanet in another star system. The giant exoplanet and its exomoon orbit their star at such a distance that the amount of radiation they receive from their star is much less than Earth gets from the Sun, but more than Titan gets from the Sun.
Thus possibly the outer layers of the exomoon's atmosphere will be could enough that the exomoon will lose atmosphere faster than Earth, but slow enugh to retain it from billions of years. But then, if the exomoon is heated only by radiation from the star, it's surface should be for too cold for humans or similar life forms.
Thus the surface of the exomoon should be heated to temperatures suitable for Earth life by internal heat, probably produced by tidal heating due to the tidal forces exerted on the exomoon by the giant exoplanet and by any other large exomoons it might have.
And possibly the lower atmosphere of the exomoon contains enough greenhouse gases like carbond dixode and water vapor to retain a significant percentage of the tidal heating, so that the upper atmosphere is not much heated by escaping tidal heating - but not enough of those gases to make the lower atmosphere unbreathable for humans or similar beings.
And also see my answer to this question:
I believe in the later article there is a discussion of the proper distance for a habitable exomoon orbiting a giant exoplanet.