For CI there are a number of drop-in commercially-available options. You can make it a staged migration, first trivially migrating the build runners, followed by the more complicated integration test and deployment runners. The CI harness itself can follow, and finally moving to a code hosting and visualization service is last.

The final step is challenging; likely the most difficult part is changing all the code references and imports. Shadowing changes would be straightforward. Training your likely 25-50 engineers to use the new code review UX would likely not take that long.

Considering the wasted engineering velocity during Github outages, it's worthwhile to do even a partial migration. Github's action runners have in my experience, been the most fragile part of the platform. Given the ease of moving build and merge queue runners to alternates, it's a no-brainer.