hey someone, did you get it fixed? having a similar issue with the fans and i believe it's to do with the PSU temp being in the high 60's. got a question myself about it and searching through ifixit for any hiding solutions.
my theory why the fans act this way is: psu is cooled by cpu fan > psu is constantly high temperature > cpu forced to go hard > psu still hot > ODD fan goes hard as a failsafe to increase airflow into the chassis. this fan problem didn't exist when the psu was in the low 40s on another imac that had the t7550, hence my speculation
i don't know why the gpu temperature isn't showing though, sorry. i've forced 64-bit windows 10 LTSC IoT 21h2 on it without a mac partition since i got it without a hdd (protip: burn a windows install DVD that uses legacy boot instead of uefi boot to avoid the nvidia driver crashes, or rEFInd to boot from a legacy bios formatted usb [i used YUMI since Rufus didn't work, and ran diskpart to clean & convert mbr before installing windows). i can only speculate it's because these imacs only officially support el capitan: that high sierra install was a hack (dosdude is your guy for that guide) and an out-dated one of that, since catalina is able to get hacked on (montrery's hack isn't working [yet?])
but yeah replace that thermal paste, just be careful when doing the gpu cause the heatsink is only held on by spring-loaded plastic barbs and not screws, so they might snap. i'm still trying to find an active nvidia heatsink with screwable insulated backplates before i make one myself using a spare heastink off a broken actively cooled ati card and some heatpipes that are long enough lol
oh yeah if you still have the mac os installed, diagnostics is launched by holding D as it boots up. if you have a keyboard with a bios switch (eg. my corsair k65) put it to position 0 (the scroll lock light will flash on and off slowly). you can also try resetting the nvram to see if that helps, some people said it has but it didnt for me. i forget the key combination but i THINK it's cmd+opt+P+R or windows+alt+P+R depending if you have a mac or a normal keyboard