Acolyte

The 1660 ti or the 2060 will run well with that monitor for 1080p gaming but if you're looking to future proof (in case you decide to get a bigger monitor with a higher resolution) you may want to go with the 2060 or even the 1080 ti.

The brand doesn't matter as much as some people may have you believe. Sure there are some slight differences, specifically in overclocking capabilities and some may have three fans and some may have two fans but underneath it's all pretty much the same. What monitor do you have and what refresh rate are you comfortable with playing at? Don't cheap out on the PSU: https://linustechtips.com/main/topic/986897-psu-tier-list-30/

More and more games these days are adapting ultrawide resolutions and it looks great but intro sequences and cutscenes will almost always be in 16:9. In Assassin's Creed Odyssey for example you can actually override the cutscenes in that game to play in 21:9 with a hex edit of the exe.

It all depends on your budget and model-wise I would match to your aesthetics and how much you want to overclock as some will overclock slightly better than others. The 2080 is more than capable if you're stretching for cash or you could go for a 1080 ti.

What's your GPU usage like in other games? You may be getting a slight bottleneck which is specific to ACO because as previously mentioned, the game is terribly optimised. I would recommend turning down some CPU intensive in-game settings such as environmental detail.

I don't think a 4790k should be bottlenecking a 1070 and the last time I checked, Assassin's Creed Origins or Odyssey for that matter weren't very well optimised. What resolution are you playing at and how much ram are you using?