Your idea about starting at a larger node is a good one, you would certainly want to debug on a cheap process.
An open sourced transistor layout for SHA-256 would seriously break open the whole competitive oligopoly that exists now. There are plenty of folks with double digits millions from bitcoin at this point that would see the benefit, so fund raising should be feasible.
This I think is the tough part. Bitcoin has changed from a cool open-source environment to ultra-greed mode. Those who have the ability to do this design certainly aren't going to want to do it for free and see some other Chinese or Russian shop take the design, kill them on manufacturing cost so the original project creators get pushed out of business, and then the takers become the next Bitmain on the originator's backs.