I had to build a reduced recoil rifle for my dad who is 74 w/neck problems. We ended up using an Areo Precision M5 set, a good stock & trigger, and putting in a BSF carbon fiber 24" 6.5cm barrel with a JP eliminator brake. This ended up being on the edge of what he can carry weight wise but works perfectly for recoil. If we had to do it all over again and it was mostly for hunting I would have done the same thing but in 6.5 grendel for weight savings and even less recoil. Personally I don't like autoloaders for hunting, my dad missed his first deer because he didn't "slam it home" when loading the first round and he struggles with the mag on the bottom even with a short 5 round mag. So make sure there is enough practice time in similar positions as the stands he will be hunting from & make sure he has the right bags for that.
I would shy away from reduced recoil ammo for any shots over 150 yards. I've ran into that problem when my kids were young.
My last thought would be that a hunting rifle is still best for normal hunting. They are lighter, they are easier to shoot from odd positions and angles and again with an effect brake (or a little less effective suppressor) offer super low recoil. 25-06 w/100g bullet is great for 300 or less (and works out to about 500) OR it might be easier to find hunting style rifle in 6.5 grendel that is threaded so you can use one of your suppressors or a brake. As "ugly" as the JP eliminator is, it is probably the most effective brake I've ever used and as a bonus it keep the muzzle down too. It is easy to see your impacts with lighter calibers.
Some reading material on brakes/suppressors
https://precisionrifleblog.com/2015/12/12/rifle-suppressor/ & Brake performance field test
https://precisionrifleblog.com/2015/08/21/muzzle-brake-summary-of-field-test-results/ from PRB. The best recoil reduction over all is Alamo Four Star and APA Fat bastard brakes followed by APA little bastard, then JP eliminator. NOTE: the top brake was 62-64% more effective at reducing recoil than the suppressor.
Recoil table for different calibers
Several calibers keep it at about 10 foot pounds of recoil. This higher velocity calibers work best in most muzzle brakes so a 25-06 at 11 foot pounds unbraked would be closer to
6.5 Grendel BSF barrel - could easy get away with an 18-20"
Good luck extending your shooting & hunting with your dad! Hope this helps.