I've heard the easily replaceable hash board before (including from Bitmain) and it never quite works out the way you think it will. Have a look between the generations and try and work out how many Bitmain generations are a. board compatible and b. heatsink compatible and that'll give you an answer as to the sustainability.
Yes. This has by far been the most challenging aspect of the design.
Varying sizes of chips and thickness.
Surrounding components.
Heat dissipation through the top of the chip or through the PCB.
Different PCB layout where chips are now in new locations.
It's no surprise where Bitmain is now: Individual Heat sinks for each ASIC.
Our solution may not be elegant, but should work.
Not ready to go into detail yet and the topic deserves its own thread.