The aluminium mounts should be a good heatsink for sure. Plus it’s a good job for the machine to do and will look great after.
The torque should be fairly proportional to the current but the heat dissipated will be proportional to power (and to the square of the current (P = Isq x R)). So by dropping the current back until the steppers are cool enough we’ll only lose a little bit of torque but we’ll get rid of a lot of heat.
BTW I’m part way through my build (still waiting on some parts), and that middle section….. wow. The more I put together, the more I see how much time and effort you’ve put into this awesome design.
As for the best way to adjust the steppers, the best way to go about it is to use the “Max Current = Vref x 2” but that won’t help with how hot the steppers get.
For the people that buy kits from you, find the best balance on VRef for torque vs temp without active cooling and give them that. If the customer wants to add active cooling and bump up the current later then they can.
For anyone buying their own steppers there’s not much you can do except tell them to use the VRef x 2 formula with their own motor current and advise them not to run the motors hotter than the glass point of the PLA (around 60 deg C).