if you can afford that much ammo.....you will likely find that one will shoot better than the others.
hmmmmm :? the small homemade block rest.......if it's solid it may not hold the gun really steady. sandbags will let the gun sink in, and it doesn't actually take much to keep it on target.
consistency is the key in shooting tight groups. grip.....hand placement....gentle trigger squeeze....etc.
reloading your ammo...tailored to what the gun likes would definately improve groups. but hunting seems to be your main objective. i'd think you could find a factory ammo to produce better than you are getting.
first, try to eliminate YOUR shooting mistakes. a good bench rest such as sand bags can eliminate most of your influence. you want the gun to almost sit there by itself.
i use old jean legs, cut off to length, tied off on ends, and filled with finely ground plastic. corn cob tumbling media, and rice are a couple more that can work, and then there's sand, which works great but is heavier.
try all these suggestions with what ammo you have first. if no results, then maybe more ammo
i understand you are already a handgun shooter, but there are tricks to get the most out of these bigger hunting handguns. they just do not handle like a 45 auto. for example....when sitting on the ground for hunting position, try resting the forearm on the side of your bent leg......or cradle it between your knees, with your back against a tree etc...........and i wouldn't expect 1 1/2" 100 yard groups from field position, unless you were extremely practiced.