Thought I had the same issue (exact same symptoms), but it turned out to be corrosion on the connection at the starter.
Well u do have the exact same problems.. ts all about poor connections resistance etc... be it in terminals , solenoid contacts, armature coils etc....all drawing far to much current and voltage drops...just at a different place.
And another issue, espec with older cars , that doesnt help, is a purple wire in particular, that may have been sorted at some time in the last 40 odd yrs... this causes oxidation of the wire strand surfaces... electricity/ electrons flow down the surface , not thru the centre of a strand....oxidised surface means resistance, voltage drop , big current
The soln to this is a relay , mounted to one of the heater bolts in the engine compartment above the starter....with a main feed added to the relay from the back of the the fuse box on the ign side...and a the purple wire as the relay trigger.
The primary cause of armiture/ solenoid failure is a far to high initial timing, causing initial ms current draws over 1200 amps, add to that the other factors and stuff starts to fail over time.
Mini starters because of how geared dont draw as high amp... even so they are still not designed for any other issues.. corroded wires , to high initial timing etc and will fail also over time.
I could not source, in a hurry , off the shelf a good factory starter a couple yrs ago.. so thru a Chinese mini in....keep in mind this is a near 11:1 compression engine with low duration and over lap cam , with cranking pressures around the 220/ 240 psi.
I did the relay around 30 yrs ago....have low initial , and VA in the idle advance.
Daily driven.. every day.. shopping , work, towing etc.
This elcheapo Chinese mini starter than I considered more or less a temp 'fix' after 3 maybe 4 yrs, still works like new.
And a side note, the Chinese HEI I thru in around 5 yrs ago.. same needed in a hurry temp soln....is still good, expect the VA diapham sucked a kumara the other day.
I will not buy chinese bearings....period.. Aussie, Korean, US, European yes.