For an on prem alternative to Jira, you can have a look at Tuleap.
We (I'm from the dev team) don't aims to re-do a "simple" alternative to jira because there are tons of them and they fall short when things to track get more complex. You can do simple (as github) you can do hellish (as jira) but unlike jira, you don't mandate one model over another (no hundreds of un-necessary fields because someone asked them once).
It's GPL, there is an entreprise plan, it's available on prem and cloud.
I'm part of the dev team so take this comment with the level of distance you want but you should have a look at Tuleap [1].
You get the customization to yet another level as you are not constraint by the fact that templates are shared across your organization. You can tweak and customize every single tracker (issue type) with it's own fields & values without impacting others and without depending on an admin for that. You can deep dive with https://blog.tuleap.org/tuleap-versus-jira-software/
If you likes gitlab and look for better planning capabilities you can have a look at Tuleap [1].
Tuleap shines with very advanced tracking capabilities and, most important, empower end users to manage it. Unlike Jira, you don't depend on a central admin to tweak you configuration, everything is at hand.
Git & CI capabilities are built-in but if you prefer gitlab for that, you will have soon an integration between the 2 tools. It's part of the next delivery [2] due mid november.
Very good agile support (Scrum, kanban, hybrid) as well as development tools (git, pr, ci).