To make a custom chip it requires one time mask fees of up to $30 million for a 5nm chip. If your volume is high then you can amortize that cost. If not then you probably go to an FPGA. An FPGA has a high per part cost but no custom mask costs and you can reprogram it in a few hours / days instead of 2 months for a new custom chip to be manufactured.
A high end FPGA already has hardened CPU blocks, USB and PCIE interfaces, and lots of other things built in. Then it has a large area of generic reconfigurable logic that you can customize to do whatever you want.
This reconfigurable logic will not be as fast as a custom chip but it is still far faster than software and can be used to implement your own CPU (assuming you don't want to use the hardened CPU or got an FPGA without them)
A high end FPGA already has hardened CPU blocks, USB and PCIE interfaces, and lots of other things built in. Then it has a large area of generic reconfigurable logic that you can customize to do whatever you want.
This reconfigurable logic will not be as fast as a custom chip but it is still far faster than software and can be used to implement your own CPU (assuming you don't want to use the hardened CPU or got an FPGA without them)