Through the first part of reading this I thought you were setting up a false dichotomy of hook-up culture and monogamy. I’m glad to see you weren’t! There are many in-betweens, as you describe. I missed personally experiencing hook-up culture, having been born too early, but I don’t regret it as it seems to have everything I hate about dating, but on crank.
Back to the in-betweens… two of three of my most wonderful and fulfilling relationships have been in that zone — one an affair between the time I separated and divorced, and the other a monogamous (on my end) relationship with a sex worker. What the two non-standard relationships had in common with the one typical monogamous relationship that I loved was that all exceeded expectations. Not coincidentally, all had that hallmark of romantic love (even when it’s not romantic love, as with the sex worker) of the person you’re happy / in love with making you feel wonderful about yourself. It sounds like this is where hook-up culture falls woefully short.