I seldom use IWB holsters anymore. I owned a respectable number of them in both leather and plastic, open top and with a retention strap/thumb-break, factory-made to hand-made and covering a wide price range.
At 509/170 I still find them uncomfortable over the course of long day/night, meaning for 8-18 hours and getting in & out of various seated situations.
Besides, I like my slacks and jeans to fit the same whether or not I'm wearing a holstered handgun, and stopped buying larger size pants to suit the requirements of carrying IWB many years ago. I sometimes like to wear thinner belts, too.
I know a handful of guys who prefer IWB and that's fine for them. To each their own.
I do prefer a holster which is designed to allow safe and relatively easy holstering/reholstering, though. I think that's a subject not fully appreciated by some folks who have never had to reholster a belt gun after an adrenalin surge and some strenuous physical exertion.
I've often thought that there were a couple of broad categories of holsters among the many types being offered. Those which were fine for carrying a gun around on the belt, without it being drawn, presented and subsequently reholstered very often ... kind of like a belt-mounted carrying case ... and then those which were more oriented toward all aspects of being a 'working' holster.
I consider this latter type to facilitate relatively easy and SAFE holstering when only one hand may be available ... and especially when an outer garment is being lifted/displaced with the same hand in order to accomplish holstering. Errant fingers, loose clothing and jacket/coat waist ties have a way of becoming caught by the mouths of holsters upon occasion, and this can be problematic (and certainly unsafe).
The significant majority of the IWB holsters which remain in my collection, just in case I find I do need one for some situation or circumstance sometime, offer mouths which remain open to facilitate holstering. Some are more comfortable than others (I'm thinking of a stiff plastic one versus the wide reinforced mouth of the leather Summer Special).
Sorry I don't have an answer for you.
I have literally felt your pain, though.