I've been told that the minimum thickness of SLC over plywood, when you factor in the mesh, is 3/8". But if I were to use a product like Ardex Feather Finish to feather the edges, could I pour the SLC onto a sunken corner only, leaving most of the plywood floor bare, and then feather the edges with the Ardex Feather Finish? Can the Feather Finish go directly onto plywood down to a true feather edge thinness?
I want to level a low corner of our bathroom and then put down an isolation membrane like DITRA, and then tile.
At the threshold, there's already a 3/4" difference in height because the old bathroom had a mud floor on top of a single 3/4" thick subfloor made of tongue-and-groove boards. After tearing up the old floor up in order to relocate plumbing fixtures and make better use of the space, I put down two 3/4" layers of plywood.
If SLC is poured over the entire floor, it would add 3/8" to the "high" parts of the floor, and make the pour in the sunken corner about 3/4" thick. This would require multiple batches, possibly pea-gravel added to the mix, and so the job would be much harder for me to do working alone; it would also make the height difference at the threshold about 1-1/2" after the isolation membrane and the tile.
Mapei Ultraplan 1 Plus can be poured to 1 1/2" thick over approved surfaces.
However, your idea about feathering has 1 glitch. If you plan on using Ditra, the installation instructions state to use modified under Ditra when laid over plywood and unmodified when used over cement. So as to not void the Schluter warranty, you see the issue.
That said, using modified thinset over the entire area isn't hurting the installation. Modified thinset over cement works but the curing time is dramatically longer. I mean days longer. If you have the time, you could feather the SLC and then lay the Ditra. Wait a few day and then tile.
If time is a factor, float the entire area with SLC.
Whatever you do, make sure you read and follow the directions of the SLC. Some require the use of a primer before installing.