“The idea is to have it be able to carry all the load of the pack when I want it to (maybe up to 35 lbs or so).”
REI has a page with a number of hipbelts it sells separately.
But given different body shapes, no single belt can be said to best meet your goal.
I’ve found that trying on packs in shops that provide weights is a way to find out which ones work best for me. Unfortunately, none of them are able to “carry all of load,” so it becomes a question of degree.
Rue the day I stopped using a modified Jackpack design and switched to Osprey belts, which are not bad compared to others in the shops. The need to move weight off the back to the hips goes all the way back to Colin Fletcher’s earliest editions of “The Complete Walker.” If you can do it, it will pay many dividends for your back.
A friend tells me some of the Arc packs from Zpacks move the weight almost entirely, but the packs are expensive and can’t afford the time or money to order packs on line just to find out if any work for me. And don’t think ordering and trying belts separately is the answer, because the pack design has a lot to do which how much the weight shifts.
What with all the factors involved, not sure there is a good way to find out what belt and pack design will move the weight best for your individual shape. Sometime, will post another pack project on MYOG that works for me, but that does not mean it will work for others. For those who do a lot of long distance hiking on popular trails, talking to other trekkers, and seeing if they will let you try on their packs, may be one of the few ways to find out what works without spending years in the work shop building and trying different approaches.
This is a tough problem to solve, and what amazes me are the number of vendors on line that show the packs only from the front, with no idea of what the suspension looks like. We have a long way to go with addressing this important issue, so thank you for raising it in your OP.