I started with Civ2 and played through most of the series. I played a lot of 5 and 6. To me Civ6 is not only the best game in the series. It's my favorite game in general.
The biggest difference is that civilizations in 6 feel distinct. In 5, all of them felt generic with cosmetic differences between them - except for Venice. In 6, with each civilization (and leader) it's like playing a different game. Preserving nature with Maori, settling tundra with Canada, cramming cities and districts with Japan, pillaging with Norway, eurekas with Babylon. I can pick the game again after two months and have a fun evening, without getting bored.
Speaking of districts, I love this mechanic. It makes sense and I like the extra puzzle and planning. I see how it gives board game vibes and I'm ok with it. Seeing the Civ series as simulation games is a mistake. They never were and they can't be. The reason for that is scale - both in size and time. The map is a grid of either squares or hexagons. Each tile has one feature and units move between them. If I zoom in, I don't get sub-tiles where I can do micro decisions. This constrains how realistic the simulation can be. On a world map, a peninsula of the size of Europe can have only a handful of cities.
Time have similar problems. Bronze Age passes quicker than the Modern Era. As a consequence, building the Sydney Opera House can take a hundred of years. And that's fine, because I pick gameplay over realism any time. Even the cartoonish art style doesn't bother me at all. In fact, I find it charming and unique compared to a boring, faux-realistic style of Civ5 which steps into an uncanny valley.
If you want a simulation approach, pick Paradox games. I tried them and I didn't like them, but I understand why they have a cult following. They still have board game elements, but the games are designed with constraints in mind. The gameplay takes a couple of centuries at most. The map isn't randomly generated, but it's scalable. The kingdoms and nations have fixed place. These decisions are consistent and make for a fun gameplay because Paradox games don't try to do everything.
It's better if Civ stays with a board-game approach. Having a clear identity is better than trying to be both "gamey" and realistic. It won't be for everyone and that's ok. For me, Civ6 is a good foundation. I'd like for Civ7 to use some ideas from Humankind, like elevated terrain, stackable armies with zoomed-in combat phase, the fame victory, and traits of a civilization that evolve over time. I'd also like to delegate parts of the game if I choose to. It would help with tedious parts and let me focus on things I find interesting.