JLC will give you 5 pieces of 52mm x 52mm 8-layer boards with VIP, 3.5mil trace/3.5mil spacing for $2, no need to violate any constraints. +$20 to get much smal

JLC will give you 5 pieces of 52mm x 52mm 8-layer boards with VIP, 3.5mil trace/3.5mil spacing for $2, no need to violate any constraints. +$20 to get much smaller vias/holes. Anecdote: I've routed a 900 pin 30x30 P=1mm/B=0.6mm FPGA with no issue on this. Use board-to-board connectors if needed to add more "layers." P=0.65mm is definitely going to make differential pairs difficult if not impossible at this trace width.