Glad you got it! If I’d done your migration all of those links would still work.
What you really want to do is to add a permalink for /questions/stop-syncing that points to the new topic – Stop Syncing - General Questions - Exalate
But if you’re lucky, and the slugs match, you can make your permalink work for most of them. Something like:
/questions\/\d*?(.*)/t/-/\1
(I can never quite remember exactly how and when to escape those slashes, but that might be right) should make
URLs like https://community.exalate.com/questions/20123204/stop-syncing turn into /t/-/stop-syncing, which will work for most of them. Sometimes the old slug and the Discourse slug might not match up. You can ignore those or create permalinks for those.