Good recommendations so I’ll just add some specifics.
For whatever emulation route you choose, I recommend using a separate SD card just for emulation. Makes it really easy to just get it set up on an SD card, the games appear and they are ready. No SD card, no emulation finagling. Keeps your ROMs off main deck storage which even 512gb fills up fast.
I like emudeck, some don’t, ymmv.
I don’t think I saw SSH recommended. I use FileZilla to transfer files between my PC and steam deck - handy for emulation files, small-medium sized games. It makes the process of transferring and setup so much easier. I like FZ cause once it’s set up there’s a quick connect so you don’t have to remember the details.
Hardware accessory: I highly recommend the Deckmate, it’s basically a VESA mount for the Deck. I attached a battery pack, mini dock, and have mounted the Deck to VESA stands. Seriously, it’s worth it.