numbers still don’t add up... Let’s assume your HF radio has a reasonably accurate representation of a S9 signal (-73dbm for HF and -93dbm for VHF+) on the S meter. Let’s also assume that the mechanical attenuator and leveling systems of your sig gen are not knackered. Then you have some 22db of frontend gain, ~5db of filter and T/R switch loss (could be better or worse), another 7db of mixer+diplexer loss. This leaves 10db of conversion gain from 145 to 28MHz. Maybe the granularity of the digital s meter doesn’t let it register 10db steps above s9, so even if the signal is there, you can’t really tell.
Do you still keep the post-mixer amp? if it’s there, you should definitely be seeing more than S9 at -73dbm.
Again, put the radio in FM mode, lower the gen level to -125dbm and listen if you can hear the hiss go down. Then compare to a good vhf radio.
VHF and above is all about weak signal reception. Unless you live in a very RF noisy neighborhood, you need to have a receiver that hears well.
btw, are those 50-ohm traces really? They seem rather too narrow...