I think, if this were a short run manga, what I'd personally do is something cheeky: I'd have the FMC somehow involved with someone, say like her brother, who hacked the Rent A Girlfriend system to make sure our MC always manages to date this one gal. She never... EVER... dates anyone else. MC's phone only sees one gal as free, and it's always our gal. Eventually she just shows up for "free" dates, then, as a sweet ending: confesses her love for our MC. The sub plot can be how much our MC finds out about the phone hacking, and how he begins to doubt it, then become more sure, etc.
I think this rental gal, as a personality, is super cute: the walking on white stones or it's lava?

I found her irresistible, even for a set of line drawings. *grins*