Hmm. Ideally I'd be aiming for 90-120 when possible (DLSS, lowering settings that aren't worth the cost, etc), but I know that won't always be the case at 4K. Do you think the bottleneck potential is significant enough to consider the 3080 instead?
First of all, wait for real benchmarks from many sources. Maybe a 3090 won't be as good as I hope it will be. I'm waiting too, never buy something blindly!
Depending on how eager or in need of an upgrade you are, I still think there is a realistic chance of a 3080Ti coming out. But that's speculation.
If 1500€ was my budget, I'd still go for the 3090 instead of a 3080 + new CPU in your case. At least for 4k. I think a 3090 will give you a bigger performance boost than a new CPU + a 3080. Even in a scenario where your GPU isn't being fully utilized.
Is it worth it to get a Founder’s Edition or should I go for a third party (probably EVGA for me) instead?
Top tier partner cards like the Trio X, Strix, EVGA XC Ultra , Gigabayte Auros Xtreme were more quiet, ran cooler and had better OC potential because of that.
That said, 20 series FE cards weren't bad and a big step up over 900 and 1000 series, those were bad.
Lower Tier cards like the EVGA black, MSI ventus, Asus Dual were rather similar to FE performance this 20-series generation. Maybe sometimes even a bit worse.
Personally I'm a bit worried how loud the new FE cards are going to be. But we have wait for reviews.