I'm waiting on the window system to get here for my 3-season Patio.
In the mean time, I'm finishing up a lot of details and trim work,
including putting down a section of the kitchen flooring that had to be
taken up, temporarily.

https://photos.app.goo.gl/Bi8qjLdnUQTjYwj69

The joists underneath this section of floor were very rotted which
helped cause a pretty good dip in the floor. Because we were pouring a
new concrete pad for the patio, I was able to cut out most of the rotted
joists, shore everything up, and replace that section with the concrete
pad.
For many reasons, it was not feasible or even a good idea lift the
joists up to take the dip out of the floor. But I didn't want the new
pad to dip with the old floor, so I had the pad poured level and figured
I'd shim the sub-floor to meet the pad.

That's where I'm at. There's a 3/4" difference in height from the left
side, 12ft. over to the right side and it's only about 16" wide. In my
mind I have 2 options...

1- Lay plywood sections down in different thicknesses, in increments
from 1/8, 1/4, 3/8, 1/2, 5/8, and 3/4, forming a ramp to level it out.
Sand the seems between sections and let the flooring adhesive fill any
small gaps. But this means gluing these sections of plywood down to the
concrete.

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?


Looking at the picture, it looks like this is sort of a bay window and
the unlevel part lays the lenght of the bay. Since this is an outside
door, have you thought of putting tile in the low spot. To gain the
depth, pull the existing floor boards out in the area. and lay a new
subfloor between the joist. then pour the concrete, and put tile on
that.

You could extend the tile a little into the room and make the tile area
a little wider.

You could even tile the room using a similar method.