My Sears diehard battery has sat for a month on my Spitfire, stock CCA and size, and never failed to turn it over at rotating speed, and fired it right up.
What you might want to look into is a battery shutoff switch--attached directly to the battery. Shuts off all ancillary drain from the battery. I don't use one, and it still fires up.
In recharging your current battery, look into a charger that desulfates the battery by pulsing. Doing so gets the--for lack of the scientific term--built-up crud off the plates off the battery, and adds life to it.
DO NOT make a strap out of aluminum or steel. One slip with a jumper cable or charging cable, and you've got arching and a rogue welder on your hands....use a proper
non-conductive hold down. Even if you have to use a marine hold-down strap.
I have never heard of having to do anything to any auto when switching from a wet cell to an AGM. Switching the type of off-auto charger orthrowing the switch from wet cell to AGM ON the charger yes, but that's it. I've dealt with AGM and Wet cells on boats for years.
Check your cables ends and the cables themselves. Old, OEM cables, replace. They can corrode internally and you'll never see it. Also the body ground attachment bolt.
Today, there's no reason a battery shouldnt be able to hold a charge for 3 weeks with no problem (yes, temp will lower its cranking ability..but that's physics).