Glad to see you again. Looks like you have a no start problem this go around.
With a voltmeters probes only on the batteries post the voltage should be above 12.6 volts. If it's below 12.6 slow charge the battery for at least 6 hours at 2 amps. I know you say the battery is not the problem, but it's the beginning of the starting system. Sure can't skip this step.
Have someone turn the key to the crank position and hold it while observing the battery voltage. If the voltage stays the same or slightly drops check the battery terminal connections and where the cables connect to the starter and where the ground cable connects to the engine
. These connections should be clean and tight. If the connections are good and the voltage drops to around 10 volts and stays with the key in the crank position the starter is mostly likely at fault. Check for battery voltage on the light green wire on the starters solenoid, (this should be the smaller wire) with the key in the crank position. If voltage is present during cranking try hitting the starter body with a hammer and see if the starter will engage. Don't bang dents in the starter, but enough to shake it up a bit. If the starter engages it's at fault.
If the battery voltage goes below 9.6 volts with the key in the crank position at any time have the battery tested with a battery tester to assure its integrity.
I hope this information is helpful to you.
Should you have any questions please let me know.
Positive ratings are greatly appreciated.
Thanks so much.