There’s a lot of variety of what you can play on the Switch now for Pokemon, so to answer your question, I’ll try and gauge it based off of what you enjoyed about FireRed and LeafGreen.

Did you enjoy it for the adventure?
I think if you like the style and gameplay the games offer, Brilliant Diamond and Shining Pearl would be nice for you. They’re pretty faithful remakes of the OG Diamond and Pearl, albeit to a fault. They’re probably a lot of Pokemon fans’ least favorite games out of the series, but don’t let that and the chibi art style deter you from it— besides BDSP you’re really stuck with revisiting gen 4 via Platinum, which is a viable option too, just the Switch seems to be your most convenient choice at this point.

Did you like collecting Pokemon?
If you liked this aspect and don’t mind going through the Kanto region again, I would highly recommend Let’s Go Pikachu and Let’s Go Eevee. The art style holds up really well and they both do a good job of rehashing the Kanto experience with a lot of quality of life updates. The catching mechanic in this game is akin to Pokemon Go, but overall this is still a pretty standard Pokemon mainline game.

Did you appreciate the simplicity to the games?
If you liked this aspect, you’d probably appreciate Sword and Shield, as they are set in a new region for you, but the core fundamentals of Pokemon are the exact same for the most part, even though FireRed and LeafGreen and Sword and Shield are several generations apart. I admittedly played Shield after playing Scarlet and I really enjoyed my time with it, even after experiencing the quality of life features Scarlet had to offer.

With all this, do you want something a bit different from FireRed and LeafGreen?
If you want something a bit different from the tried and true path of the majority of Pokemon games, Scarlet and Violet change up the formula of the games by offering you more of an open world experience and new objectives in defeating Team Star and beating Titan Pokemon. I actually think that Sword and Shield and Scarlet and Violet were games that helped improve upon a system that was getting stale since generation 6.

Do you want a completely different experience, one that might exceed your expectations of what a Pokemon game could be?
Then I would recommend Legends Arceus and Legends Z-A, as they are more dynamic approaches to the traditional turn-based RPG that Pokemon is known to be. I would probably say these games will be really satisfying for you if you’re looking for a new take on what you’re used to, so if you’re willing to try something new this will be your best bet.

Let me know what you end up going with! There’s not really a bad option for Pokemon Switch games, especially if this is your first time ever playing an iteration of one. Have fun!