On Sun, 28 Oct 2018 13:09:50 -0500, -MIKE- wrote:

snippage
2- Pour a self-leveling mortar bed. This seems to make the most sense
to me, however, I've never worked with it before. Also, on the left
side, it's going to be paper thin where it meets the existing pad which
is already even with the old sub-floor. Won't that area be prone to
cracking and breaking up? Would I need some sort of mesh fabric to
reinforce it?

Let me know what you think.

There's a product called LevelQuik, a self-levelling underlayment, which
should do just what you want. Also, the bit on the left side will be
sandwiched between your finished floor and the old subfloor, so I
wouldn't worry too much about any cracks being noticed on the final floor.

Cheers,
Colin