Menthol, would you care to explain to me what makes you feel Personus Quantum is “better” than MOTU’s mid/higher priced interfaces (excluding their really cheap alternatives) such as their 828es?
I don't really think the Quantum is a better interface than the 828es; in fact, objectively speaking, it might not be quite as good. It's definitely not as flexible and feature-laden. However, it is I believe the better bang for the buck and is considerably cheaper and simpler, both of which appeal to me. Plus, it's Thunderbolt 3, not 2, which saves an additional $50 by avoiding the ridiculously-priced adapter you would have to get to hook the 828es up to your iMac. Overall, I think the price difference is significant and I'm not convinced I would use all the bells and whistles on the 828es. With the money you save you could then opt for a bigger SSD on your iMac.
I saw a long discussion on Gearslutz about the 2626, which included comments from owners of both the 2626 and the 4848 who said they could detect no difference in sound quality between the two. The reviews have also been stellar (Sound on Sound: "The newest addition to the Quantum range offers a plethora of I/O and excellent latency performance for a very reasonable price.") Perhaps the biggest vote of confidence in the 2626 is that it's so damn hard to find -- Presonus can't keep up with demand.
I can tell you're an agonizer, which I'm sympathetic to, in that I can go down that road as well. I relate to both your general rumination on the wisdom of getting a 2020 iMac, as well as the issues related to using it for audio. But sometimes it's just better to make the decision and then NOT LOOK BACK. I am quite certain the iMac and either audio interface will help you do amazing stuff if you can stop worrying and make the decision. It is HERE NOW AND VERY USABLE; it's not worth it to worry about what may be available in one or two years. You'll just be stuck in an endless loop.
For what it's worth, I ordered my iMac last week and I'm getting the Presonus, so I'm putting my money where my mouth is. There are small compromises, but few of us have unlimited funds to spend on, say, a Mac Pro and a $2500 interface, so this is what I'm going to do. For me, it was time to act. Of course it's not my place to tell you what to do, but I hope I can help at least put you in touch with some of the cognitive baggage that is holding you back.
So consider this a pep talk and a friendly nudge. ?