I have to agree. I'd really like if there were more infinitely replayable tasks that match the narrative theme (heists/robberies, making money, buying businesses, spending money, repeat). I appreciate the work that went into even the smallest of mission cutscenes, dialogue, and scenarios, but outside of the missions there isn't a strong through-line with these characters being thieves. You're pretty much just a normal dude and can go hours not doing anything related to the theme of the game including busywork that doesn't give enough of a reward to tie back into that theme.
I get that maybe GTA Online is more dedicated to "living the life" and I might get what I want from that, but I would've enjoyed to see more of that from the singleplayer characters. Would've gladly traded Golf/Tennis/Yoga/etc. for consistent side work + mini-heists for each character that match their stories. It's already perfectly set up with the repo + meth business storylines. Michael could had his own thing as well.