> The head of SRE championed Gitlab CI. I resisted this idea because I, the relatively inexperienced manager of a nascent team, was daunted by the prospect of trying to supplant Jenkins, Github, and JIRA all at once.

I think this is a really big scoping mistake. There is a clean glide path, and I have no idea why you'd think you need to replace Jira.

You can use Gitlab Runners in Github (https://docs.gitlab.com/ee/user/project/integrations/github....). Then with your mirrored repo you can try out the Gitlab features and realize it's better to use Gitlab for your repo if you're aso using it for CI. Having switched back and forth over the years (with tens of engineers using the repos), it's really not a lot of work to migrate between Github and Gitlab.

Finally, if you like Jira, just use that? Sure there are advantages to having everything in one place, but if you're already using Jira+Github then you'll get an equivalent experience with Jira+Gitlab.

I think this is one of those things that if you had actually prototyped the migration, you would find that it's much easier than you thought it was going to be.