Hello.
Were it me: I would just try your existing power supply. As long as it's not poor quality, which yours isn't, then there's miniscule risk of your rig or GPU becoming damaged. I would try it, and then buy a $25 USD wattage meter for peace-of-mind. As a convenience, a Multi-Outlet Extension Cord plugged into the wall, then my wattage meter (Kill A Watt P3 P4400), and my my power supply cable plugged into that watt meter. The extension cable makes it convenient to look at the wattage figure next to your computer, instead of at the outlet. The "multi-outlet" extension card allows the wattage meter to lay somewhat flat on the floor, unlike a single connector extension cord.
-- OR --
You could wait until you get your hands on a RTX 5090, which might take some time. By then, perhaps Corsair will have released their 2025 HXi lineup of power supplies, which have wattage metering and other electrical analytics built into the unit. Then you could bypass purchasing the wattage meter and extension cord all together:
https://www.techpowerup.com/330908/corsair-shows-off-2025-rme-series-and-2025-hxi-series-psus-at-ces
Also, Corsair's existing HXi lineup is still available for purchase:
https://www.amazon.com/CORSAIR-HX1200i-Modular-Ultra-Low-Supply/dp/B0CTR6PK19/