many individual miners are probably in their 20s or lower 30s at that oldest
Don't kid yourself.
I've mined for years and I'm 66 years old.
As soon as I need to put the heat back on, I'll be firing up the ASICs again.
I know that I am making broad generalizations - certainly, I did NOT mean to imply that everyone falls in the age group that I specified. That is the reason that I surmised with the qualifier "many."
Second: Are you really a seasonal miner in that you believe that your getting a second purpose (heating) out of your unit justifies running it at faster speeds during the cold season and slower (or NOT at all) during the warmer seasons. Personally, I would have speculated (again) that these machines have such a short lifespan (due the seemingly ever exponentially increasing hash rate) that you would want to continue to run them while they are still profitable and with the passage of time they are going to become, more or less giant paper weights.. or maybe ONLY heaters b/c they are otherwise running at a loss.