OK, finally got some time to do some diagnosing.

The battery is/was fully charged.

I've got +12V at the starter relay's battery feed stud (large stud).

I've got +12V at the "Ignition" (in the Start/Crank position only) stud of the relay.

NSS lead, on the starter relay, shows 0 ohms in "Park/Neutral" and infinite ohms in any gear.

The starter relay's output wire (down to starter solenoid) receives +12V when the ignition key is turned to the Start/Crank position, telling me the starter relay is OK and functioning as usual/expected.

All this being said, I believe the starter is at fault.

I have a newer Denso starter (PN 56027702) which I believe should fit/work.

Can I use the original starter mounting bolts with the newer style starter?