@Marcii I just bought the same 7900 xtx and had a Seasonic Prime 850w titanium power supply and my computer was shutting down due to over current protection circuit cutting in (likely from transient power spikes from the GPU). The Nitro+ was drawing just over 400w (as reported by AMD adrenalin) when gaming at stock. I did use the automatic under volting feature in the adrenalin software and that reduced power draw to just under 350w. Was good for a week before it I got another shutdown. After getting another couple of shutdowns over the next two days I just went out and got a new power supply last weekend. I went total overkill with the new psu, got another Seasonic, one of thier new Vertex GX models at 1200w (and here I thought my 850w was overkill when I bought it over 6 years ago... lol) that support the new ATX 3.0 standard. Since installing the new psu, I haven't had any issues and I turned off the under volting and switched the bios back to it's OC mode. I supposed I could have tinkered with power settings to reduce power draw even more but I would rather cranked it up to 11 and pull more power than turn things down, but that's just me.
On Sapphire's website they recommend 800w minimum power supply for the 7900 xtx nitro+. But my 850w psu didn't work for me. In my case (pun intended), I have 4x140 fans, 2x180 fans, plus the 2 fans on the noctua DH15, that I'm currently using, plus rgb lighting with my 5800x and 32GB (2x16) of 3600 ram (with rgb of course) and two nvme SSDs (and that's not mentioning any peripherals I have plugged into the computer, all of which is drawing power as well). Not sure if you will have success with tuning down the power on your soon to be new nitro+ with your current 700w power supply but would be fun experiment to try . And don't forget that every fan, rgb light strip, ram stick, etc will all draw power. When I installed my new gpu, I also put in two more sticks of ram and pulling them out afterword reduced how often I was getting shutdowns when gaming. So I was on the knives edge for power, so why I went with the aforementioned psu. My recommendation would be to consider a 1000w or greater power supply, preferably using the new ATX 3.0 standard. Though I suppose 850w psu using the new standard could work too since it would allow for transient spikes of double the power draw (which would be 1700w on a 850w psu). That just my thoughts