I doubt resolution is a limiting factor LCoS (transmissive ones are lower resolution typically though, but you could build reflective) phase modulators are available in 4k resolution (and maybe even higher). And I don't know if you need that much resolution because the regions you're trying to focus are quite broad, in fact I suspect the resolution of your phase modulator would not limit resolution but the max distance between focused regions, because it would set the max phase ramp you can achieve.

Loss, i.e. equivalent aperture is a different matter and I think this would imply quite a light loss.