I have a system in which the main habitable planets are on the many moons of a super-neptune. The ice giant has not migrated inwards, but rather the habitable zone has expanded outwards due to the red giant phase. However I need this red giant phase last long enough that life can form, evolve into an intelligent form, and colonize neighboring stars, all starting from that super-neptune system. Red giants are unfortunately not particularly known for their longevity, and 1 billion years (as our sun would last) simply isn't long enough.
There is the potential for life having a head start here if it formed in a tidally-heated subsurface ocean like that of Europa, so the red giant doesn't need to last 4 billion years+, as life would only need time to colonize land and then land life develop intelligence and technology. However the longer it lasts, the better.
My first guess was that a lower mass star would also have a longer lasting red giant phase, However I found out it was impossible for low mass stars to even become red giants; so the problem remains that its too short-lived.
Magic and aliens are not allowed as this is the source system in which the first life developed. At least in this galaxy.
I have also considered a couple options for older universes, such as making the central star a red dwarf that evolves into its hotter phases (Helium dwarves and blue dwarves for example) and thats how its habitable zone expands, however that necessitates a very old universe to accomodate the time for a red dwarf to evolve that far, which is inconvenient considering I want really massive stars to still be present and dominating the color of galaxies.
or by having a high metallicity star. However I haven't been able to find much information on how stellar metallicity effects the lifetime of stars and their red giant phases, so I don't even know if this would lengthen the phase.
Edit: High metallicity stars last shorter according to the answer by HDE 226868, so that option is out of the question