Hi everybody. I'm extending antique 1.5" oak into a former porch (now living room) with reclaimed antique 1.5" oak. Previously it was just OSB at the wrong height so that's gone. I've putting down new 3/4" plywood subfloor everywhere I can, but since the old porch was a little sloped, within a foot or so of the existing flooring in the rest of the house, 3/4" would be too thick. I can go down to 1/2" etc in places, but I'm still going to be left with all kinds of odd gaps and shallow indentations and things (see photos).

What techniques are available for coarse/fine subfloor flattening in odd/narrow/shallow spots, prior to hardwood? I've ruled out anything self-leveling as it's not meant to be nailed through. I've heard of shimming with roofing felt, but that seems like a terrible idea for inside a living space. I've already cut a boat-load of custom pine shims, but in these tiny areas that's becoming less feasible and much more tedious.

Here are a few photos, in various stages of completion:
https://photos.app.goo.gl/m4XMxZ6grU7BSZRf8

Wood shingles. Roofing felt is used indoors. A cleaner product is Aquabar B which is a moisture retarder.....HD has it. If you have an area that dips in the middle put down layers of the Aquabar or felt and build up the area.....like a topical relief map. 5/8" is minimal thickness for a plywood subfloor.

If you use asphalt saturated felt, which comes in several thicknesses, you can build up the surfaces to within an eighth inch over six feet using a straight-edge. I recommend covering the felt with craft paper like Aquabar. Aquabar has a layer of wax/asphalt between layers of paper to keep water vapor away from below the flooring. It is pretty tough so it helps to even out the surface keeping the flooring in alignment even if it is not perfect.

Thank you both, I had not heard of Aquabar B and will check it out. I know less than 3/4" plywood ain't ideal, but there are old floorboards underneath a layer of asbestos tiles, which hopefully the 2" cleats bite into well enough. Short of scrapping the project, are there any alternatives?

I think that you ought to use twice the fasteners as recommended when you have to use 1/2 inch ply. This would be within an inch or the ends and every 4-5 inches along the board. This will make the surface more solid. In the past I have come across flooring without any sub-floor, just close spacing for support as in a sports floor. Surface nails every 6 inches, if necessary.
The cleats will not split the edges like old finish nails can.

Ok, thank you very much for mentioning this.