Have you looked at self hosting sourcehut (https://sourcehut.org/)?

Yes, but all those services have the same main problem: a single point of failure. They also don't work offline.

I believe that storing the issues in plain text in git repositories synced across several git servers is more robust and future-proof, but time will tell.

Having a simple storage format allows me to later on export it to any other service if I change my mind.