Margie,
Hate to write this here but will withstand the haters.
You're asking a lot of an AGM battery with a MK3 that you've fought long and hard. I know the electrical system has been fixed - needed it pretty bad. All those bad things hit the battery hard when starting and didn't help while recharging. Yes, fixed but damage done. A good (even cheap) AGM in a non-electric start will last. In a MK3 you need a lot of "juice" to start and when down just a little on voltage it takes even more current to spin the motor. This is especially true of the original more and cables.
Fire Extinguisher ready: Flame on regular flamers:
I prefer these: Shorai LFX18A1-BS12 18AH or LFX18L1-BS12 18AH but most places are out of stock. IMHO, the 14AH version is just good enough and the 21AH version is overkill in IMO.
These should be a good substitute:
https://a.co/d/aLEsoel but check the dimensions to be sure it fits your battery box, they are bigger than the Shorai battery I use. And I would also buy a Noco Genius charger unless you have a Lithium-aware changer. I use the Noco Genius5. It knows Lithium, AGM and Lead Acid. It can be a smart trickle charger but there's little to no need for trickle charging a Lithium battery.
I have forgotten what regulator you have - if PodTronics or original Zeners, remove the fuse if leaving the bike parked.
Don't leave any batteries in freezing weather. If cold out, and using a Lithium battery, turn the bike on, turn the headlight on, wait 30-60 seconds. turn the headlight off and start. That warms a Lithium battery and greatly increases its "power" for starting.
BTW, the last three Shorai I bought were $25 less at Wallmart.com than Amazon.com and came just as fast.
With the battery I use for e-start, you can start and turn off and start 10-20 times - you'll never do that with AGM batteries. For non-e-start bikes I use 8amp AGM only.