Hey
I don't imagine the 1-click Rails install will work easily with FatFreeCRM out of the box. The baked in versions of Rails and Ruby are quite different to what FatFreeCRM is currently using. I haven't used DigitalOcean for a while, but that's my initial impression.
You're not really restricted to what you install on a DigitalOcean droplet, so there's no reason why you can't install Ruby, Rails 6, etc, but you'll have to either do it all by hand or set up an Ansible / Puppet / etc script to pull up your server into a known configuration.
If you are just starting out in hosting you might be better off looking at a PaaS like Heroku, Fly.io or similar. They remove a lot of the headaches around hosting and I'd recommend using something like that - unless you are really keen to learn how to configure a server from scratch.
Having said that, it would be nice to get FatFreeCRM running under Rails 7. I'm interested in having a crack at that, though I think someone might have already started.
Hope that's useful.
Cheers,
Tom