I just learned that PROTON_ENABLE_NVAPI=1 to enable DLSS got changed to PROTON_FORCE_NVAPI=1 in Proton 9.0 (including Proton Experimental). Just in case this helps someone in the future.
On another note: Feel free to let me know if you've got a good way to keep up with necessary/useful launch options. I've simply been having a look at what people use on ProtonDB. But I've come to the conclusion that a lot of the posts there use outdated or simply false launch options.
And apparently the plan is to have explicit sync ready for the next major driver version (v555).
From the discussions on Github and Gitlab it seems the work for that to happen is done. The changes in the necessary packages (Xwayland, Mesa?) just need to be merged and the the Nvidia driver 555 needs to be released. It hasn't been that long since the previous release 550. So I guess it is going to take a bit of waiting still.