I struggle mightily with my 2018 mini. If my upstairs gets into the 80s, it just shuts down. I run security spy on it, have it on its side with air all around it.
Can you share some numbers from Macs Fan Control when it's idle?
Also can you share some screen snapshots from Activity Monitor (CPU tab, sorted by decreasing % CPU) when you think it's idle?
I've never had a thermal shutdown on my Mac Mini 2018 and that includes running it flat out for a week+ at a time with all CPUs going and a recent period where it was >85 deg F in my home due to an AC issue.
Thinking of upgrading to a M1 or 2 but this mini has 32gb of memory and 1 tb drive in it.
M1+ will of course be faster and run cooler. Unfortunately as you know matching RAM capacity gets expensive at the high-end with Apple Silicon. Getting close with a new M4 w/24GB and 1TB will run $1200. An M4 Pro w/48GB and 1TB will hit $2K (at which point, buy a Studio) with shipping dates in September. Worse, Ebay prices for M1 and M2 w/32GB even higher than Apple's price for new M4...
If your current system otherwise meets your needs, I would try to fix it. What you are experiencing is not normal. Mine is currently running 50 deg C while streaming video with a several background apps running. Thermal throttling not to mention thermal shutdown doesn't really start until 100 deg C. Add 10 deg to my number to account for cooler ambient temperature (though I am probably only 5 deg C less than you) and you aren't even close.