I think your mapping should turn out fine this way. It could get a little comlpicated with the loopback and syncing with Ableton, but since trying this you are not a newbie, you should find out all the quirks as sometimes Ableton can be a bit on the picky side with configs.
I agree the reloop has extremely nice turntable like jogs, with a good profile and rollback. I did not like the SX jogs (which I guess are the same on the SX2) that mutch, they just had too much drag for me.
However I also totally love the jogs on my MCX8000 or the MC 7000. Denon just delivers a notch above reloop quality wise and I really love those controllers.
Now if you really need Midi-time-sync between Ableton and Serato, be sure to read the articles found on the Serato.com forum.
I actually do not use time-sync I just do it by ear 🙂
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