Anyone have ideas about how to fit this third party driver board into a 2021 (m1) imac? (link to board)

It’s hard to say for sure just by eyeballing it, but I’d agree with your assessment that there’s probably not enough space in that chassis.

My suggestion would be to abandon the idea of reusing the iMac chassis and instead design and build a bespoke case. It wouldn’t be nearly as pretty unfortunately, but that seems unavoidable.

It could be as simple as a box comprised of cuts of thin wood or plexiglass/acrylic, but for a more apple-like result there’s relatively inexpensive metal cutting, deburring, and bending services that will cut sheets of aluminum and other metals to spec which should make it reasonable to produce a custom chassis with rounded corners. In the latter scenario I’d recommend getting mounting structures for the driver board, PSU, and display panel 3D printed in a heat-resistant insulating plastic like PETG which can then be mounted to the aluminum panel with threaded heat insets melted into the structures.

As an aside, I share your frustration with mass market targeted displays. None of them seem to value DPI, which means that instead of optimal size + resolution pairings like 20”/21” 4k you get awkward ones like 24”/27” 4k which require non-integer scaling to be usable (which has gotten better, but is still anything but trouble-free). It baffles me that there’s no manufacturer selling that 24" 4.5k panel in the iMac as a dedicated monitor, because it’s really very nice.