Starter heat soak

Assuming the electrical system is adequate;
The voltage may be dropping so low because the starter is working so hard. All that energy is going to magnetism and heat, because the starter cannot spin.
I would repeat the HOT-test with the spark plugs out, to see how much of the work is to overcome compression.
After that, if still low, I guess I would back off the rocker gear, to eliminate the spring-pressure.
At this time, all you got left is pistons and rings, plus whatever is attached to the crank.
If the voltage is still low, sub in a different starter, charge the battery and start over.
That's how I diagnosed mine.
BTW, my Optima Battery doesn't seem to have a lot of Rest-voltage, but it does seem to crank very well at 10/10.5 under load. I had to get used to that.
On mine;I don't know if the tightness was skirts or rings. There was no evidence on the skirts, but I figured a half a thou couldn't hurt.Actually, I think I might have gone a full thou, that was year 2000. I didn't keep notes,lol.
But I opened up the gap a lot. The KB-recommended gap-factor on my KB107s was .0065 Which comes to gaps of .026. I went with their towing factor of .008, which comes to .032. I installed Plasma-Moly file-fits, and that was the end of my problems. So after that, I increased my Thermostat to 180 then 195, and finally, she now runs 205 measured at the hottest part if the stathouse. Still no problems; click-vroom/vroom.