Long form conversation time. This is a pretty straightforward engineering problem. You're trying and failing to suspend a weight on the side of an inverted cone. You have three levers you can pull: 1) weight 2) friction 3) geometry.
1) Weight
You've found that pocket carrying a mousegun can work. Somewhere between the weight of your aluminum compact frame and a polymer subcompact (inclusive) will be a tipping point between successful carry, and a feeling of being undergunned. What other guns have you tried? Maybe something like the P365 will be in that sweet spot for you?
2) Friction
You've seen that tightening your belt
can work, with drawbacks. For free, you might try punching a hole in your current belt, to see if a half-notch wider would still work with less discomfort. This is basically tweaking the Normal Force part of that equation. You might also consider the friction coefficient - can you put something between your body and the belt/holster that makes it stickier? (More on that in a second.)
3) Geometry
OWB carry puts a weight outside the belt (obviously) and forces the belt to resist a twisting motion. This is why we care so much about having a stiff belt. It can take a marginal arrangement, and cause it to fail. You noted that pocket carry is working with the smaller gun, how big of a gun can you pocket carry? Trying another (stiffer) belt may help, as it could better resist the torque.
Moving to IWB might help resist the twisting, but also (like
@jeffC says above) you may find a holster with a friction material that improves the hold.
Moving to a different location (4 o'clock, Appendix) might help, as the edges of the belt can be allowed to grab and bite against you in the area with the smallest radius (your hips).
This reminds me - how high do you wear your belt? That is, on your hips, or above them? If you move higher, you will be cinching around the soft part of your body, instead of essentially trying to compress bone. This might require pants with a longer "drop" to ensure that the crotch is in the right place WRT your belt line.
Have you tried a belly band? Worn over a tee shirt, the neoprene should hold up pretty well.
If none of these work, we get to the "drastic" options. Obviously a shoulder or ankle holster takes the weight off your hips. The shoulder rig can we worn free, or braced to your pants, but then you're wearing suspenders. Ankle holsters are slow, but nobody is looking at your ankles, so printing
really doesn't matter.
If you're worried about the shoulder rig being too obvious, have you tried one of the
concealment shirts?
This is probably the most expensive solution, as you'll need several of them, and they'll wear out faster than a regular holster, but desperate times call for desperate measures. That said, you can get a similar effect with a belly band worn high in the armpit. Regardless, you'll likely have to change the way you dress to support this higher mode of carry.
Good luck!