Don’t bother with a portafilter pressure gauge, it will just read exactly the same as the pressure read by the machines gauge, regardless of the flow rate. The problem with the flow rate could be a blockage it could be an empty heat exchanger where the group would be too cool as well which might affect the temperature of espresso, all the very slow flow rate affects the temperature of espresso.
90 g in 60 seconds is nowhere near an acceptable flow rate for a rotary pump it should be seven or eight times that maybe more.
I’d recommend you check the brew circuit and group head for blockages initially, if you can’t find any, run a good descale through the brew circuit, a really good descale.
I’m guessing your machine reads the same brew pressure against an open group.
