Appreciate your response - especially that you remember a bit about me despite only having a few posts.
"Serious amount of time" certainly doesn't mean 2-3 hours - the absolute max would be 100 balls, probably around an hour, maybe slightly more but not by much. Totally agree that productivity of practice drops as you go past a certain time, I am careful to avoid going past that point - often leaving the last few balls if I don't feel it's worth it at this point. On the other hand, it does sometimes take a long time and a lot of terrible shots before something suddenly "clicks", so I also look to make sure I persevere even if it feels unproductive. It's a fine balance.
Your description of the swing sounds (hopefully) like what I'm doing. It also makes sense that driving would cause more pain, as the hips-faster-than-shoulders movement is more exaggerated. So maybe it is just a case of sticking it out, always stopping practice when it hurts. Also worth mentioning that the pain is always gone within an hour of stopping, not to return until I swing a club again.
Just on what you said about "machine-gunning" - I actually find that approach can be useful to me if a cerebral approach isn't causing any progress. It is possible to overthink so thinking about the shot is doing more harm than good, I find that sometimes rapidly smacking a few balls one after the other bang bang bang bang bang can help instinct to kick in and help make a breakthrough. Worst case scenario, it lets off some frustration and I can take a breath, and go back to swinging thoughtfully with a clear head.