You'll be fine. I used to shoot them with 3" #2 steel duck loads out to 40-45 yds. If you look at the lethality tables, #5 lead and #2 steel hold lethal penetration to about the same yardage, 45-50 yards. Now, you will want to pattern the gun to see how tight those patterns are at those...
I'll say this, you were pretty much dead nuts on that 300K estimate in the OP. I thought for sure the stone would be the most expensive but the bighorn blew it out of the water.
Hmmm, it's more personal preference. I one tripped 3 mature mule deer bucks (110 lb ish pack weight) and half a bull elk plus camp (140-145 lb) this year with a Terminus. I previously used EXO and Kifaru frames, the SG Terminus handles weight better than those. Can't comment on the new models...
You're gonna need more capacity than 7000 cu in. I would look at the SG Terminus 8700. I have the 7000 version and it checks all your boxes, but you need more volume than that.
You can in 2 scenarios;
1. You are a beginner weight trainer.
2. You are on steroids.
Any other circumstances, no. You may be able to get stronger as you lose weight (to a point) depending on your individual leverages and your ability to adapt neurally.
Also, maintaining cardio while...
I'm a 300 yd zero guy with my 270 Win. With a 126 HH @3250 fps it's point and shoot out to around 340 ish. I dial past that, if I have time, but am comfortable with holdovers.