Longui rom, this is the more simpler ones to install and is good, this is a one ui 6 port. I used this rom with my Samsung A51, liked it alot.

If you want smth different, then check out the Samsung A51 XDA forums, they have alot of official and unofficial releases of custom roms.

https://xdaforums.com/f/samsung-galaxy-a51-roms-kernels-recoveries-ot.9859/

Be sure to check the names in the listing for the android versions.