Well after being unplugged for the past few days, I took out my after market RAM and started it up. It always starts up after being unplugged overnight, so this is neither good nor bad news.
I am running my backup script to hopefully get a new clone copy. If that finishes, then I will try to use it for some light surfing/editing to see if it stays on, but I am letting the backup run without anything to interrupt it.
you've gotten plenty advice/opinions here, so at this point I'm sharing my current thought process on my future computing plans, because you and I have some similarities:
. almost identical iMac spec
. like to keep our computers for a long time (my iMac replaced a 2008 MacPro ...)
. we use LR/PS - though I believe from your photos and some of your posts you are a "heavier" user than I am ...
I just in the last few weeks upgraded my iMac from Mojave to Big Sur, part of that was to replace my 1 TB external SSD (that's where I keep my photos) with a 2 TB one (Sandisk with similar to T7 specs) and a new backup strategy. This way I am hoping to get another 2 or so years out of my iMac ...
one of the key takeaways from the above was the recognition that external SSDs are FAST, eg copying 740 GB of photos onto the new SSD took ~ 20 min, and it's not TB3, it's USB 3.2 (whatever spec #).
Buying a new high-end iMac 27 (or whatever the new model will be) will set you back ~3-3.5k$, my eyesight is not getting better with age, I will never go back to a smaller monitor ...
Technology is evolving very fast and we are at crossroads now with Apple's M silicon ...
So here is where my head for my future computer needs is right now: most of my data will continue to live on external drives (photos, music, movies, currently totaling 3+ TB), my "documents" take up less than 50GB, my LR index/preview currently takes up ~150GB. So an internal SSD of 1 TB will be sufficient, might even do 512 depending on cost.
Get an excellent monitor, 27" or larger, at least 5k - ~ 2k
Get a high end Mac mini M2 (or whatever name) with min 32GB and max 1 TB SSD, at least 4 TB3 ports ~ 1k
Going down that path will enable me to "upgrade" the computer rather sooner, if it ever fails, it's going to be simpler/cheaper than an iMac. Performance even with external storage will be much much better than what I have today.
Also, this week I got a new MBA M1 with 16GB, 8core CPU, 8 core GPU and 512GB to replace my MBP (got a great trade in offer that I did not want to miss out on,
initial impression is that LR/PS are pretty fast ...
Current M1 based Macs are Apple's "entry level" Macs, Adobe did not create LR/PS for "entry level" computers, they created them for professional and high-end users and that will NOT change, they happen to perform decent enough on Apple's new "entry level" computers.
Sorry for the long post, and I am going to end here hoping that I left you with some food for thought ...
Good luck with the iMac, I really hope you can get it back to work so that you don't have to make a "rush" decision on replacing it eventually.