How to bring existing data into fresh GitLab installation?

Hi !

we’ve got an on premise GitLab CE installation running for over nine years.
It’s been updated a few times in the early days, then migrated to a docker-based installation and since then been updated a bit more regularly. At the moment, we are pretty up-to-date with a 13.11.x.

The problem is:
Several of the recent updates failed and we first hard to tackle the failures down and fix them manually, e.g. by renaming database indexes to the expected name.
There are also some speed problems that seem unique to that GitLab instance.

So, we assume that the installation is messed up for some reason and we’d like to start with a fresh installation, of course while preserving our data.

What is the best way to achieve that?
Is it a good idea to make a gitlab-rake backup with the old installation and restore it in the new one?
Or will that also restore a messed-up database schema etc.?

Best regards and thanks