Just got the RG35XX H! What now?

So got the RG35XXH (due to arrive tomorrow or the day after) and bought a 128GB SanDisk card.

Having a look at what people said in multiple threads, I intend to go with the Modified Stock OS, single SD setup. I'll follow the RetroGameCorps guide for the most part and use the Balena Etcher on MacOS but I still have questions:

• The modded stock OS doesn't have games....so should I copy the ones from the stock card? Even if I take the time to get ROMs...is the boxart often included or do I need to get the images from somewhere else?
• If tomorrow I want to change to Knulli/muOS, can I flash it to the same card? What's the process to clear partitions and format SD card?
• Any other recommendations to enjoy the device to the maximum from the get go?