I just did a copy/paste of the following phrase from your initial post- - - -

However, reading voltage at the starter it will show 6.25 volts or so.

Is this voltage reading being taken during cranking? If so, it tells me that the starter is not drawing enough current for some reason. A slow-cranking starter draws HUGE amounts of current, because the counter-EMF that's developed by the armature spinning at normal speed isn't being developed. That drops battery terminal voltage dramatically. I'd strongly urge a swap to a different starter as a diagnosis tool. Battery terminal voltage during normal-speed cranking should be somewhere in the 4.75-5.0 Volt range due to current load on the battery, even a new, commercial-grade 6 volt.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!