Bugzilla. Seriously.

Can be easily extended and unneeded fields can be disabled via template.

We use it since 20 years at a 6000 heads company and it's totally fine.

You can try Ekso:

https://ekso.app

Fully self-hosted, dockerized.

We made the pricing reasonable -- YMMV.

https://www.openproject.org/blog/open-source-jira-alternativ...

I haven‘t used it myself, though

Honestly the best issue tracker I've used so far is Phabricator (now Phorge). It would be a bit weird to use it just for issues though, and tbh I'm not sure I would recommend it as a forge because it has approximately no CI support.

Redmine, maybe? With or without Redmine X.