This sounds like the "Bendix" that engages the starter to
the flywheel has stuck, failed or gotten so poorly connected
that it can't honk the solenoid in, but can lightly spin the
rotor (not engaged = no load).
Every winter morning I would have to go and thump the
starter motor on my '66 Mustang with a 2x4 (kept just
for the job) , before it would quit doing the same.
I would begin (with ground off the battery) R&Ring the
fat red connection to the starter and the starter solenoid
ends. Ruling out electric feed quality, any change? If you
are up for pulling the starter, might inspect the action on
the bench (crusty? rusty? loose / broken connection to
the Bendix solenoid (or what takes its place - old school
is my frame of reference).