I have the base M1 MBA, and it's been very reliable. I didn't rush out to buy it. I waited for any "showstopper" bugs, didn't see any, and I can't complain one bit about the performance. I will ask, if you had 40GB of RAM in the last Mac, did you have any idea what kind of memory pressure you had? I suspect 16GB would work for you just fine (I edit 20MP RAW on just 8GB), but I don't want to presume. If that checks out, your programs work in M1, preferably native, and you can handle the drop in screen size, I'd say just get the M1. There's always something better coming, but unless you anticipate your workflow changing significantly over the next few years (like editing much larger RAW files from some yet unpurchased camera body), it's just a stress you don't need to worry about. These M1 iMacs just launched, so you're probably a year away from something better even coming out.
Apple has a super-forgiving return policy. 14 days, no questions asked, free return shipping even. Yeah, there's Amazon, but I returned something to them recently, waited 2 weeks with no refund or status updates, and had to ask customer service. I got the refund, but for a company that can deliver same day, that's pretty sad. Don't get it from B&H or Adorama if you might return it. They stopped accepting returns on any opened computers sometime recently.
And to not answer your question, I was suspecting dust as well. If it clogs the works, the system will overheat and shut down. Based on the time from boot to crash, it's a very real possibility. But maybe the Mr can save the machine. Props to him for doing the research and buying the parts--but that's a car guy for you. 😛