This was my exact experience with GTA V. I loved exploring every nook and cranny and soaking in every little detail. IV was actually a bit of a bummer to me because the world really did feel a bit empty outside of missions. To this day, nearly 10 years later, there are still times where I'll fire up GTA V single player just to get out there and explore and play around a bit. I also found the storyline more than suitable though you'd never find me calling it flawless or anything.
As it pertains to GTA VI, the matter-of-fact nature with which everyone throws around 2024 and 2025 release dates is quite a bit disheartening to me, as likely as those prognostications may be. This is one of my all-time favorite series and—relative to gaming—it's kind of tough to know that I'll need to wait a quarter of my life for the next iteration. If we're lucky, many of us seem to be planning on 10 years between V and VI and quite possibly more. That's insane.