If you want to minimize subflooring buildup, take all of the subflooring off, install ledgers on the sides of the joists, install 3/4" ply on the ledgers to fill in so it is level with the top of the joists (running the normal way you would with the grain), then put at least 1/2" layer of ply on top of that.
SLC can be poured thin, but over plywood, many manufacturers want lath and a miniumum of 1/2" over the top of the highest layer. It is hard to get the slc to flow if it is enmeshed with the heat mat...it needs some height over it. Think pancake batter, not syrup. The slc will be brittle, if it isn't bonded well, it will crack and debond, then it'll sound like stepping on Rice Krispies...not good!
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters) and Longtime Forum Participant.