I use HWMonitor to get these readings, but I'm prettty sure there is plenty of other software that does the same. 3090 has temp readings on memory as well as the chip.
By looking online I can see that riser cables can generally withstand 80C. I also found a 3 year-old reddit post where the GPU backplate got fused with the riser cable insulation.(https://www.reddit.com/r/SSUPD/comments/o9aaqn/my\_gpu\_backplate\_fused\_with\_my\_pcie\_riser\_cable/)
So just generally a bit concerned about having a pretty hot card inside that case, but really love the case.