I have NEVER regretted spending too much on a computer. I have regretted spending too little. Three or four years after buying my current 2010 single 6-core Mac Pro , I deeply regretted not buying the dual 6-core when my tasks and software changed. That's a good comparison to the M1 Max vs M1 Ultra (dual M1 Max). Software always gets hungrier for compute resources over time and who knows what new things you will be using this Mac to do 5 years from now, so why not give yourself more headroom for the future?
I used to agree wholeheartedly with this, but it has also become... more complicated.
In support of it, I will say I have generally tried to max out my computers (to a reasonable degree), particularly for peripherals I couldn't change out - usually processor and graphics cards. It paid off for my current 2015 retina iMac; I was comparing notes with another user of the same photo-editing software, almost the same computer except that he didn't max out his graphics card. We determined that his choppy performance contrasted with my flawlessly smooth performance due to differences in the graphics card. What was it back then, a $200 upgrade? Choppy performance when adjusting sliders would have driven me to upgrade almost instantly; that $200 "investment" allowed me to use my computer frustration-free for an additional three or so years.
I used to upgrade systems about once every 3-4 years for the performance upgrades; my current computer is about seven years old. It's the longest I've ever used a single system. But it's not just that the rate of technological advancement has been slowing a bit: I've been slowing down in what I do. I don't push my computer like I used to, and I don't get to spend as much time on it as I used to, either. Actually, I'd happily keep plodding along with this computer, except that it's on the verge of no longer receiving major operating system updates and I could benefit (quality of life - want, not a need) from newer features.
I'm stuck on configuring my own Studio. If a 2015 system is doing fair by me, I already know the Ultra configuration is ridiculously overkill - something I'd probably jump at for $200, but can't justify $2,000 for. The base Studio itself is possibly overkill, but the Mac mini introduces compromises that I don't want to have to work around. I can afford it, and this is where I'm with you on regretting spending too little. Buy once, cry once, I say. But the graphics upgrade now is about cores, and not an architectural or GPU-based RAM upgrade like it used to be. RAM used to be an easy one when we could do it ourselves, but being locked in now makes it harder. I run 32 GB now and can push it on occasion, but it's fine most of the time... but I'll probably take the 64 GB upgrade just to be safe.
But that's all thinking for someone who basically runs with the same system until it's not working well for them anymore. OP sells his systems to jump between upgrades; the calculations on that are different. I'd imagine that you'll have a smaller subset of people willing to spend so much second-hand on the Ultra configuration, and would have an easier time selling at the Max price point, but I don't have much experience selling for multiple thousands of dollars.