How big a gap with foam sealant, installing a door in a lightweight concrete wall

[fender stratocaster]

I'm going to install an exterior door with a frame in the barn, it's a leca wall so I thought I would use lightweight concrete screws. Is that a good choice?

Since the hole in the wall is much larger than the door, I'm thinking of building a wooden frame beside the door and attaching a plywood sheet there, then plastering the wall (and insulating from the inside). Does this seem okay?

And the last and most important question, when I install the frame with lightweight concrete screws, how large of a gap should I leave between the leca and the door frame to achieve a good result with the foam, so the foam doesn't squeeze the frame?

 

[Strip]

The size of the gap matters less, you should still stabilize the door frame so it can't deform. Measure the nozzle of the foam gun, then you’ll have a minimum measurement at least