AFAICT their license does not fulfill the OSI definition and is therefore ineligible for inclusion in the main F-Droid repo.
They have their own F-Droid repo:
https://app.futo.org/fdroid/repo/
The repo is also pre-included in clients such as Droid-ify, which I use anyway because I like it better than the official client.
Which one of the three I suggested?
FUTO is definitely a bit more resource hungry. Try Heliboard, you need to add a separate library to enable swiping, but it’s definitely way lighter.