Thanks for setting out the FP versus injector duty cycle considerations. That was an interesting read. The external AFR option, usually mounted remotely to the fuel rail is also a space hog. Prefer something neat and OEM like the Bosch 4 Bar FPR. Might work for you if your current direction of travel with more injector duty time etc etc doesn't play out
https://www.bosch-motorsport-shop.com.au/fuel-pressure-regulator-4bar
Probably wont work in your application but this site offers a Bosch Regulator housing adapter that is designed for the Bosch FPR. There are probably similar offerings out there.
https://www.kainjection.com/product-page/bosch-regulator-housing-adapter