Well, what to start with when you have to take care of a new Jira instance?
In our experience, the best way is:
👽Collect the system specification (who owns what, what is an infrastructure (for Data Center), what plugins are installed and - a tricky question - are all of them in use, how many users do we have in user directory/using the system in one minute)
🔬Check through the Atlassian documentation if the infrastructure is fine for the amount of users/automations you have
🕵️Ask project owners for their opinion on how it goes with Jira - are they satisfied with the performance, are there unresolved issues in place, what are their expectations about the next automations you'd provide to them
That's it, now you own the technical side and the service development roadmap. When this steps are made, you could plan the next steps - probably some addons should be decommissioned, an infrastructure should be updated and, of course, you shouldn't forget about providing new possibilities for end users.