Series 7 is the sweet spot (I'm using it currently). If you already have the series 7 and didn't value the incremental value of series 8 and 9, I see no reason to upgrade at the moment. You also ruled out Ultra for the size so won't go there as well.
Apple has been doing the incremental changes, so their products seems very similar to the previous generation, but would be more worth upgrading if you keep the product past a few years. Why not just wait a little bit and see if series 10 has a feature plus the incremental upgrades in between that you like that may make upgrade worth it?
I for one loves the majority of the new changes of watch os 10. I know it's very subjective though, so not going to argue about it.
If you want good integration with iphone, Apple watch is definitely top of the list. The other smart watches or fitness bands have their advantages, but mostly not related to the integration with iphones.
I say wait it out to see if future Apple watches will suit you better. I'm not the type of person that cling onto an old OS, but if you really really don't like watch os 10, maybe get a series 8 that comes with watch os 9?