I don't know your tractor whatsoever, but like a few here, am quite strong electrically. (just in general...not Kubota electrically neccesarily)
All indications are that not (enough) voltage (and thus current) is reaching the starter itself.
This can be caused by several factors:
Key switch, Relay, starter solenoid, connections to either/and/or the solenoid or the starter, BOTH ends of BOTH cables +/- going to/from the battery and starter/solenoid. LAstly, bad cell in a battery that reads 12+V but breaks down under load.
I recall (some of) the other thread on this, but honestly if this were me, almost the first thing I would've eliminated was bypassing as many elements of the starting circuit as possible.
Understanding you are not super strong electrically this may take some 'splainin'
but as strange as it may SEEM you need to work backwards from the starter.
ie: Use your truck....running.....jumper cables.....neg jumper cable to frame of tractor near starter and positive wire to the starter lugbolt itself (not even the starter solenoid at this point in time) The tractor battery is NOT in the circuit at all . Unhook it.
This SHOULD work if your starter is OK. If no joy, you can use some gentle persuasion with a small hammer or big wrench on the heavy parts of the case, just in case (no pun intended). Don't kill it.
If no joy, the problem is mechanically or electrically within your starter. If it starts with just voltage from your truck to your starter, move back one element.
Next, do all the same stuff, but to the starter solenoid. Again, I haven't the faintest clue about your model of tractor but generally speaking it's a small cylindrical switch located either on top of, on the side of, or very close to, the starter. It's just a BIGGIE remote switch that passes a lot of current. All your key and relay do is "tell" the solenoid to connect 2 contacts together to pass lotsa current to the starter. On some cars, you can literally bridge the gap between 2 large bolts that 2 big wires of the positive lead are connected to with a big screwdriver and a big spark will happen....again.....never even looked to see what the BX25D has for a solenoid, so don't take that as neccesarily applicable in your case. Tapping it gently with a wrench can sometimes bring elation if most of the issue is dirt, corrosion etc. ( don't beat it within an inch of its life or anything crazy) Solenoids are not usually known to be tough guys like the thick casing on a starter. Gentle is the key here.
If this works, go back one more element....maybe the relay is not playing nice and passing the "message" onto the solenoid to operate. We're dealing with very little current up here and even a small amount of
corrosion under a crimp or half the strands in a wire broken will/can, cause the relay to chatter and decide if it wants to play nice and pass the message onto the solenoid to do its thing or not.
Then the relay by your dash. Does your relay pop into a socket of some kind? (If so, pretty easy to pop it out, clean connections and pop it back in) If sealed or black plastic , hard to see what's going on in there. Myself, I'd dremel/cut it open if I couldn't pry the case off and then burnish the contacts, but that's me. I'd do things with power supplies and multimeters we won't get into here. One thing you COULD do is just tap it a few times lightly/gently with the plastic/handle end of a screwdriver in case the issue was mechanical, not electrical. ie: rust or corrosion on the moveable parts within. If it's on the end of a bundle of wire you could jiggle it around gently to possibly reveal a loose wire or crimp, corrosion or bite marks from Mickey or Minnie./
These are just a few ideas.
Make SURE you know what's happening if you try to start the tractor with any of these ideas when you're not in the seat. ie: in neutral, parking brakes on, wheels all blocked/chocked up or in the air, etc.
Don't run yourself over !
Oh, and btw, every single time you crank on that thing and it doesn't starts makes your (new) battery all that much more weaker for the next try. You'll need a 120vac-12vdc charger of some kind
(or)
Once do you do get it started let it run an hour or two to even HOPE to get the resting voltage back up to levels that are useable for NEXT time for some cranking. The glow plugs and starter both draw LOTS of current. Current that's not put back into your battery very quickly
This all assumes your alternator is working. That'll be problem #2 once you get it runnin'...because...how did the previous battery go dead? (I'd want to know that)
Good luck