Phabricator works great at the scale you are talking about. With a single monotonic issue number for your company (like Apple's Radar). You have to ditch GitHub totally though, and use Phabricator for code review or the wiki too to get the benefit. Avoid it's repo hosting and CI component (harbormaster). The thing you will miss most is the web based file editing.

My advise would be to deploy: Phabricator (issues, code review, wiki), Buildkite (CI - with your own AWS runners), gitolite (repo hosting).