I had one hydro lock but fortunately the piston with fuel must have been at TDC so no clunk and no damage, Took the plugs out and it sprayed fuel all over when it turned it over,
Maybe I missed it but I don’t see where you said you checked the grounds (beyond cleaning connections last fall). It’s easy to overlook the simple things sometimes.
The daily High on Friday this week will be 5 below at my home in south central Wisconsin. I’m in Florida for a couple weeks right now and it’s 73. Not looking forward to returning home on Sunday!