any way to help here? what is needed next? the ticket title seems a bit broad.

As for logic: how are the ticket numbers being converted?
Considering that bug 1 must become task 1, and that RT is still in the 4 digits, maybe it would be enough to make RT(X) become T(X+90000). Then the phabricator-native tasks could start at 100001.

Bug 1 will not become T1, as explained in T857 and https://lists.wikimedia.org/pipermail/wikitech-l/2014-October/079251.html.

RT and Bugzilla tasks will be migrated starting by the first task number available in phabricator.wikimedia.org, and will not attempt to keep any resemblance with their original ticket numbers.

In T376#17859, @Qgil wrote:

RT and Bugzilla tasks will be migrated starting by the first task number available in phabricator.wikimedia.org, and will not attempt to keep any resemblance with their original ticket numbers.

Yet, we are attempting to redirect the old URLs to the new ticket numbers by keeping a bunch of redirect rules that map old to new numbers?

That would be T41: Set up redirects from RT URLs to Phabricator