I'll add my own two cents here, source: I work in the field.
1. Technology
The reference to 'near future' tech is problematic in this, as 2-3 decades is the typical timescale of planning & constructing (in the case of James Webb more like 4 decades) a major observatory or space mission, given current funding horizons.
Given those timescales, technology in large observatory projects is frozen at a given, defined point (otherwise you'd keep exchanging and updating components) and then this frozen level of tech is used for the remainder of the existing mission (this is particularly true for space missions, ground observatories i.e. next generation interferometers and ELT's can be build much more modular).
Because of this, large observatories always lag a few decades behind compared to the newest standards, which are e.g. used in smaller science missions or top-secret intelligence satellites.
So with this disclaimer, it's clear that your mission will essentially use the technology which is in todays spy satellites, and be a typical factor 3-10 better (as it is usually per generation of telescopes) in angular resolution, signal-to-noise S/N, spectral capabilities, etc.
This factor 3-10 is a huge oversimplification, and particular for the light gathering capability (or sensitivity, or S/N) the size of your telescope and/or number of array telescopes play an overwhelming role. There, in principle you can just spend more money for a larger telescpe or more array elements, but the tech again limits whether you are actually able to analyse and use all the incoming data for an improvement in S/N.
2. The science
For what we can actually do with the data gathered, we have to distinguish between discovery and characterization. Discovery is only a factual statement, that something is there, while characterization is going beyond and actually saying what is there, how much etc.
The dividing line between those two modes of science is roughly the S/N ratio of the data you have: If you're fishing in the noise, and you barely see the signal of an Earth-like planet in the transit data, then you can only claim discovery. Your data is not good enough to say anything about its bulk properties or the existence of an atmosphere or other things we can today only vaguely dream of (like shape of continents etc.)
What determines the S/N in the end is the technique you are using and what object you are looking at:
- A self-luminous young giant planet, far away from its host star will have great S/N in the infrared, its light can be directly fiber-fed to an infrared spectrograph. This will yield a lot of spectral data, which can then be fit to determine temperature structure, abundances of atoms and molecules in the atmosphere. Currently, that's still a noisy job, but the next generation of ELT's will do a fantastic job at that.
- A small, terrestrial planet, close to its host will have terrible S/N, and even discovery of atmospheres on those planets is extremely hard currently, and only for near-by stars, not your 1800 Lyr far away star. The ELT's are going to improve "discovery of atmosphere around Earth-like planets" into occasionally doable (given our understanding of how technology will evolve).
However, to do what you propose, you'd need your telescope size and tech to move into the mode of "routine characterization of atmospheres around Earth-like planets, at large distances", which would probably need another 2-3 generations, large-scale international funding and collaboration and world peace. Also those future scopes would presumably so insanely large that you couldn't take them on your generation ship ride, you'd have to keep a commlink with Earth and take the hit that your data doesn't become better, the closer you get to the target.
3. Detection biases
If you want to travel to a far-away 'perfect' planet, you want to first make sure that there is no other good candidate much closer, otherwise you're wasting a lot of money.
Your proposed 1800 Ly, which translate to something like 430 parsec are currently in the outer range of sensible exoplanet detectability by (transit) surveys. You would need to first step up your survey machinery dramatically to be sure to cover all stars that are closer than this with transits, and then another dramatic step up to cover all planets with direct imaging.
Transit surveys are biased towards the planets which happen to orbit on the line-of-sight, whereas direct imaging surveys would have a much higher completeness. However for this, novel coronographic techniques would need to be developed to block out the star (e.g. Starshade) and deployed in large numbers.
Given world peace and all surplus funding in the next half century going into exoplanet science, this would be doable, but seeing the state of the world at the moment, this is not what is going to happen.
