Other thing to consider is that you probably won’t be 310 pounds anymore in 6-8 months. So that is also something to consider in your decision, and possibly give you motivation to reach your goals!
But… I also echo the comments on if a greenbroke horse is the correct mount for you when are you just getting back into riding again. Would it be more suitable to find a been-there-done-that horse?
What are you wanting to do? Trail ride? Shows?
The 20% rule is NOT a rule … it is a very, very generalized guideline that doesn’t consider the points already mentioned above. So please do not worry about following that to gospel! A 120-pound rider that is POUNDING on the horse’s back with every stride because they can’t balance themselves is much, much worse than a 250 pound rider that is moving gracefully and smoothly in rhythm with the horse. And the “20% rule” doesn’t touch that.
Just for fun…
Many years ago, I was at a gaming show and ran the sack race with two different people. This is a timed event where you must make a full circle around 4 barrels while holding a gunny sack between the 2 horses. Fastest time wins. You cannot drop or let go of the sack or you are disqualified.
First video: One gal I ran with probably was around that 250 pound mark. I don’t know for sure how much she weighed, but that’s just a guess. For reference, my horse that I am riding is 16.1 hands. So her horse probably was around 15.2 hands and he was a stocky-type build – suitable to carry her build. Despite her large size, she was a great rider (still see her all the time but she no longer rides due to back issues). Her bobble around the far side of the arena was not her – I remember my horse bumped into her on the first turn. They paid out first through third place. We barely took 4th place by a few tenths of a second.
Second video: In contrast to the other person I did the sack race with that day. A little girl on wicked fast little pony. We won the sack race and took first place. She’s maybe… 80 pounds? Maybe? (and a hell of a little rider, I may add) So there was a huge weight difference (and yes, horse difference) between my two partners but it only amounted to not even a full second on time. I want to say we were 10.7 sec with the heavier rider and 10 flat with the smaller rider, if my memory serves me correctly.
So maybe not relevant to your situation OP, but that little pony was ridden by adults too and he was successful well into his 40’s. Never had a problem. And I know the heavier rider did not abide by the 20% rule and that horse too was ridden for many, many more years and was a great horse.
So probably … just throw the 20% rule out the window. That might be better.