If you are only gaming then there is no need to cool the backplate, 74C Tjunction is a very good temperature already, stock 3080FE and 3090FE can reach like 90C-100C Tjunction.
You can increase the radiator fans speed and it would lower VRAM Tjunction along with GPU temp too.
Overall just enjoy your 3090, there is no need to do anything else beside playing games
Its even lower in games but I have not reached 100% load in games yet. I play at 1440p and I think the most stressful game I play at the moment would be Star Citizen.
It got to 74°C tops with mining (daggerhashimoto). My radiator fans don't need to be cranked, I use a MoRa 360 in push/pull with 8x 180mm fans. It has its own loop, I got three separate loops now for my CPU, my 2080 and the 3090.
And yes, these temps are fine but I'm the kind of guy who chases every improvement possible for hardware even if it makes no sense at all.
Which is why I did this:
I placed some heatsinks on the spots where it does not create clearance issues. Glue is your typical thermal silicone stuff which I found works well enough and has a very nice and strong bond while not leaving residue when you remove it.
I'm letting it set at the moment, will update later if it changed my thermals at all.
EDIT: Dropped the memory mining load temp by ~2°C to 68°C. Backplate feels noticably cooler to the touch. I know direct contact heatsinks would improve things a lot more, but I don't want to void my warranty yet.
Comparing this to regular air cooled models and some of the other watercooled ones I'm happy. Seems to be a very healthy temperature.
What block are you using? My watercooled (bykski) 3090 memory junction temp maxes out around 75c ish after a four hour+ session. It's been rainy here so nothing to do but game I guess.
I got the inno3D Frostbite card, comes with a block made by alphacool. Once it runs out of warranty I will probably replace it with a block from watercool or aquacomputer.