What kind of fan, radiator, fan location, engine, trans, coolant type and mixture, shroud, etc.
A thermostat does a specific job, and when it works right with the rest of the system, you won't have any problems.
It could also be that you're getting bad thermostats. I have a friend who went through 5!!! before he got one that worked right and didn't overheat the 289 in their Targa Newfoundland race car. They changed everything else, and kept going back to the thermostat. Changed the T-stat one last time, after changing everything (including pulling the engine), and after that last t-stat, never had a cooling problem again.
its bleeding the air out is my main concern, run the vent pipe into the rad over flow and run a non pressure cap, then fill it through a moroso filler neck in top hose which has a pressurised cap. pics are not great but you can see my top hose is not ideal but without stat in cools fine with it in we seem to get air pockets
Man, you need to re-route your hoses. You've got too many high spots for air pockets to fill.
Air will insulate the block from the water, so you're right in wanting to get the air out of it.
I always drill a small hole in the lip around the thermostat's flange to bleed air out while I'm filling the system, and it seems to work well.
Get a different t-stat housing/water neck, take all the extra bends out of that top hose. The radiator has a cap on it, but you're running a no-pressure cap? I had a hard time following your description.