I don't want to leave the old OS on it (you have to provide a Samsung ID to install anything).
LineageOS? CalyxOS?
This will primarily be a device to play around with. Might have some lightning wallets on it, but hopefully I can avoid installing anything that needs Google play.
I have the same one. I installed LineageOS very easily and works really nice.
https://xdaforums.com/t/unofficial-rom-t585-lineageos-18-x-11-0-64-bit-treble.4511073/
https://community.e.foundation/t/unofficial-builds-galaxy-tab-a-sm-t580-gtaxlwifi-sm-t585-gtaxllte-sm-p580-gtanotexlwifi-and-sm-p585-gtanotexllte-for-e-os-s/52910/29