So, my understanding is that the route now lives and runs well in T:ANE?
Accordingly, forget the TS12 version for now and import the route only from T:ANE.
Recommend rebuilding any sessions you might have in T:ANE for this route again in TRS19 from scratch. There will be too many potential scripting problems otherwise.
Try the CDP option first. As this is a compressed file format, it should (in principle) be more compacted and come in under the 500Mb limit.
Have you worked out what exactly is responsible for such a large final route file size? Either the route itself is enormous, or you have included some extraordinarily inefficient assets somewhere there.
Was it built on a DEM project, perhaps? Can you optimise it by removing any unnecessary route layers/ assets?
Suggest use the Compact Route option in T:ANE before you try to export it. (Save a FULL copy of the route elsewhere safely first!).
Alternatively, opening the route for edit in T:ANE and then importing it into the Editing Folder of TRS19 might be your best course as suggested above.
After submitting the edits in TRS19, you'll still have quite a few clean-up items to deal with, including downloading any DLS dependencies and substituting older T:ANE built-ins with their TRS19 counterparts.