So, my general opinion is that you don't want to run a PSU at more than ~70% load. Sure, it's technically possible to do so and should be fine, but I prefer a larger buffer between rated output and used output. So if the 5090 ends up at 600W, that's 60% of a 1000W PSU right there. Add in another 100~200 watts for the CPU and rest of the PC and you're at 70% to 80% load. If you have a high quality PSU, you should be fine. Anyone using an 80 Plus Bronze PSU, I'd start to be concerned.
Your sig says you have a Zalman Wattera 1000W, which from what I can tell launched in 2015. PSUs do age over time, so if that's an up to date system components list in your sig and the 5090 does end up with a 600W TGP rating, I'd seriously consider buying a new 1200W or larger 80 Plus Platinum PSU.
And it perhaps goes without saying, but I'm still very concerned about pulling 600W through a single 16-pin connector, no matter way Nvidia and PCI-SIG say! Those are relatively small wires and connectors and that's a lot of power. I'd be happier with dual 16-pin or even quad 8-pin. If the 5090 has a 550~600 watts TGP, I will be shocked if there aren't more melting connectors down the road.