I recently did a bare metal migration from Gitea to Forgejo using NixOS, maybe this info is useful if you use SQLite (which I believe is the default): the SQLite database filename for Gitea is gitea.db and for Forgejo it’s forgejo.db so I had to do a rename. Before renaming I ended up with an empty Forgejo instance. Either way I hope you figure it out in the end. Good luck!
I use Obsidian, you have mentioned it and it’s not self hosted, but for me that depends on how you look at it. I use it in a folder that’s synced to Nextcloud, so I consider the data self hosted markdown files.
The viewer, i.e. the Obsidian app is not self hosted, but I consider that just a client used to view the data so it doesn’t really bother me.
I recommend Ruby on Rails. I am biased with 17 years of professional experience, but it has batteries included, end to end everything you need to build, test and deploy a modern web application. In my opinion Ruby is the most pleasant language to read and write. But try to compare many stacks and see what you like best.
In my experience (self hosting mail since 2005) signing up for SNDS does factor in. Although last time I had trouble with delivery to MS, my hosting provider Linode's support also helped out by contacting MS back channels on my behalf.
The biggest problem I (rarely) have is when whole IP blocks end up on a ban list that MS seems to really trust.
That said, fuck it, I will keep fighting the fight and self host my mail like a stubborn old git :p
I've recently switched my entire self hosted infrastructure to NixOS, but only after a few years of evaluation, because it's quite a paradigm shift but well worth it imho.
Before that I used to stick to a solid base of Debian with some docker containers. There are still a few of those remaining that I have yet to migrate to my NixOS infra (namely mosquitto, gotify, nodered and portainer for managing them).
