I noodled around with Phabricator for a while and it's slick in many ways but extremely clunky in others. For instance, importing a repo that exists in another VCS is done at creation time in Gitlab as part of the flow - it's not a completely different step.

In Phabricator, you create the repo, you turn off its access URLs, add URLs for the repo you're mirroring, set them as a mirror source, and then turn the access URLs back on. This is about a 5-10 minute process per repo and it's all clicking around in a UI.

Have you looked at the rhodecode.com for svn/git?