After 8 years I finally took the time to upgrade my websites framework from rails 4.2 to rails 7.0.4 and I'm amazed how easy it was. I expected a lot of rewrites like back then between rails 2 and 3, but I only had to change 2 lines of code, remove the hard coded dependencies in my Gemfile, add two config files and rewrite my systemd service file. So most things outside of the code itself.
Sadly I don't have any automated tests, so I had to test everything manually which took quite some time and I probably missed some bugs which will show up over time. I also wasn't able to test all the connections to the outside world like Twitter, Mastodon, indie-web, etc. but those are not really essential to the functionality of the website anyway, so I'll test it over time.
Anyway, great success! And now I am not afraid of extending the functionality, I really want to add a picture album functionality and move over all my 50 albums and 4000 pictures from flickr to be able to host them myself instead.