Yeah, I don't think the sleeping thing will work here. I just listed the Cyberpunk interactions as examples of buffs. The buffs ED players would recieve would need to come from other activities.
I think it could work. I know some are highly averse to any sort of survival aspects being brought into Elite, and I can understand that to a degree. However, I think there's a way to incorporate some of these things in a way that can add to the atmosphere/immersion without making Elite become a press E to eat/drink every five minutes type affair, which I agree would suck. Having some quarters on your ship and fleet carrier would allow you to log off the game by doing a
Bagpuss, and also you could choose to rest during the supercruise, which might make it worthwhile to take a mission that actually takes you off to a relatively distant location. Especially if the rest does give you a stamina/health buff to your character and you're on your way to an onfoot mission. Food buffs would be good too, though best incorporated with expanded crafting/synthesis, which could bring a lot of other cool things to the game too but branches off into a separate topic from just interiors.
As far as other gameplay elements that could make interiors and also EVA worthwhile would be the ability to actual carry out hull repairs manually, which could be of great use to explorers. Of course, there are limpets that can do that, but that's an extra module which might be fine for an Anaconda, but for for a DBX that slot might be put to better use. One of the things that is both good and bad in Valheim is the wear and tear mechanic - I don't suggest lifting that straight into Elite as I think it's a pain to keep doing over and over on a too regular basis in that game, but the fact that you see panels that need repairing could be applied to sections of the outer hull of the ship which would then allow for you to use a hand held tool to repair, I guess Space Engineers might be a better example on how a tool could be used to carry out such things.
I also think the above could be worthwhile in the Thargoid starport defence scenarios - you don't want to leave the instance but you're banged up enough that you can't run the gauntlet back to the Starport, so you retreat to a temporary safe spot to carry out some repairs or decontamination. This could be external and internal if your modules need fixing. Again, AFMU is there for that but I think the buff idea could make it worthwhile to deal with as suggested by OP.
Overall though, I think the biggest hurdle to ship interiors in the way that everyone views them is technical, Star Citizen is shown as the example of what it should look like, and I agree, but I think Star Citizen also shows that it's a tech that still causes glitches that are too large to ignore, and I wouldn't want those sorts of things in Elite until it's basically bulletproof. Empyrion gets around this by only allowing you to pilot the ship by sitting down, and having to come to a standstill before you can move around, other players can then sit down and fly the ship and you'll go flying if you're not, but I think that approach would work fine in Elite but with the caveat that everyone has to be seated before take-off, which if we think about it, is exactly what happens when airplanes take off in real life.
Supercruise could have a mode-switch, like a seatbelts on/off sign on a plane, where the ship maintains the current vector and velocity with a warning sound, like that scene in Star Wars where they are warned of the approach to Alderaan, which could be tied into the supercruise assist module. Being snapped to the pilots seat if you don't make it, in the same way that happens with the Fleet Carrier is acceptable to me as a trade-off.
Anyway,,, suffice to say, I think free-roaming interiors that work in a space game scale are still a game-technology that is not ready yet and I would rather wait till it is before saddling Elite with that nonsense. But, I think there are ways to provide a good degree of freedom of movement around the ship without going down that path that would add a tremendous amount to the game. IMHO of course.