I've got a "server farm" under my desk with a few JBOD boxes and some large USB drives, somewhere above 30TB on my MacPro.
My "solution" to the bottle neck is to work locally then save globally.
I, too, work on massive files, many over 1GB and many quite close. It's easy to create files with lots of layers and with hard drives so large and cheap, it's not time efficient to continually have to down-size files to save a little space like it was decades ago when drives were expensive, small, slow and unreliable.
My internal and close-by drives are SSD and spinning RAID-0 for working files and once ingested, metadata installed, processed, winnowed and processed, I take my time and simply back-up at that time. In the short term, everything is backed-up using TimeMachine so there's redundancy in the system.
For my library/archive, they are all on spinning disks, several years on each and then as drives get cheaper/larger every few years, get combined into newer disks, the others stashed off-site with multiple copies.
For your work in progress, why not back to a Thunderbolt drive, and every week/month, back-up to large, off the shelf but smaller and slower than you wish, available/inexpensive USB 3 drives? Backing up even at the end of the day on slower drives while you do something else works well for me. The last back-up to a fast drive took several days and was simply a matter of dragging folders from one drive to a newer drive every evening and then turning out the lights for the day. I figured it was also a good way to make sure that if the drive did fail, it did it early on and while under a pretty good and long load.
My eventual plans are like you with my "last computer" as I'm no longer hacking nor wanting to spend a bunch of time with the overhead I once did decades ago. I want a computer like my 2009 MacPro that will last for a decade with little effort and great reliability.
My guess is that I'll take the drives out of my "server farm" of JBOD boxes and simply migrate/consolidate on larger/newer/faster Thunder/USB boxes, but we'll see when the times comes. I'm almost thinking that even an M1 iMac in one of the fancy colors may be all I need, but there's plenty of time to see what happens in the next few months as what I have is working swell.
You've got lots of time to hash things out and if your system is working well, bide your time!