Seems to me, you’re literally doing everything wrong, while admitting you know you’re to blame, but effectively refuse to make any serious changes. At least not any that would actually help. Like why not try something other than square taper? Why not spend money on a more race specific crankset that will be better suited to the torque you choose to inflict upon them? Why are you using 110bcd MTB cranks? Why are you using specifically 170mm length? A machines crankset is literally less strong than a forged one. A steel crankset will flex so much that you’ll never get all your torque to the ground. Effectively nulling the heavy gear you want to ride, making all of this pointless. Also, your current frame has flex, that’s guaranteed. Introducing more with a steel frame may be helpful for the life of your frame, but it won’t do any good for your crank.

my advice is to switch to a non square taper bottom bracket and modern race crank. You could do an outboard bearing bottom bracket and road crank, with custom chainring. If money isn’t an object, my specific recommendations would be the shimano dura ace 7710 track crank with octalink bb, or the sugino 75 DD ‘direct drive’ crank, as it has outboard bearings. Both of these choices have larger diameter spindles, and they are specifically designed for track racing. You may need to address your frame though for clearence issues.

simple fact is, if you’re going to purposely ride that way, and put down serious torque, you need the proper equipment. That means spending serious money. You don’t want to do that? Either change your gearing, or keep breaking things. But that will cost you more money in the long run. I’m not trying to be rude, just blunt, because I’m genuinely trying to be helpful. You’re to blame. Anyway, let us no what you eventually decide, and good luck. Cheers.