GBA emulation is the best. All the games are meant for small screens and modern hardware has absolutely no issues with emulating the GBA. It has soooo many games. You can also get ROM hacks (most common for Pokemon games), which are fan edits to games. Some of which are such massive edits that they're basically entirely new games only using the existing framework for the style of gameplay.

Only downside is controls. Touch screens are simply not as good as physical controls. I don't recommend trying any games that require fast paced and precise controls, as you will hit wrong buttons plenty. You can get Bluetooth controllers to help with this I think, though I've never personally tried them. I've also seen "case controllers", which go around your phone and make it sorta like a Switch form factor.