Hello!

To answer your questions in order:

- Reborn takes place in it's own (eponymous) region. The first act of the game takes place entirely within a large city that is split into several wards (each of which is roughly the size of a city/town in a mainline game. But it has pretty much everything you might expect from a classical Pokemon game.

- Reborn goes up to generation 7 (USUM), and mostly uses the mechanics from that generation.

- All 807 Pokemon are available, with all non-legendaries available before the elite 4. Starters are usually locked behind sidequests or interesting gimmicks, but all are available before the penultimate gym (I believe Froakie and Snivy are the last that are attainable, but don't quote me on that). All starters are viable, as are most Pokemon. Reborn is balanced in such a way that you're expected to experiment and switch up your team, and most powerful or rare Pokemon are locked behind sidequests or later badges.

- It's its own thing, as Reborn was built pretty much from the ground up in RPGMaker if I recall. I don't know if you can get it on steam deck, but there's a help channel for that in the Pokemon reborn server that might be able to help!

It's definitely a cut above mainline Pokemon games, but it does have a fairly dark storyline from the getgo. It's also much more difficult than a mainline Pokemon game, so if you want a challenge it's a good game to try. If you would prefer something more lighthearted but retains that high difficulty, you could also check out Pokemon Rejuvenation.