wrt her 'enjoying' the sex...just wiggle around more swimmin, hit those sidewalls. It will somewhat compensate for your lack of friction argument. I feel a similar predicament in that I don't have a ton of girth, but you can compensate by positions that make things tighter (her legs together), literally just going in at angles to hit sidewalls, and work them hips/grind it out.
I can't get behind either of your arguments that your size isn't big enough to please them (over 5 seems fine?) and it doesn't create enough friction to please you (ur still nutting, and a girl that's just grinding on me doesn't even have to have much penetration and it still feels awesome). Look into practicing more forms of tantric sex, foreplay, toys, kinky ****. Don't see how sex could ever lose its luster for either party even if you were 2 inches - there's just a million more options than simple D in V. I guess the fact that you care zero about her getting off changes a lot of things - that's always a pretty major goal of mine with any normal girl.
I have a bit above average US dick by your count and have had more than a couple non-slut girls tell me candidly that it has been the best sex of their life, while at the same time admitting its pretty skinny
gl out there