A RTX 3090 would help...:P because of the 24GB of VRAM...:w00t:
Higher details means larger textures and that takes a lot of GPU memory (VRAM). An indication is given on the iClone Info overlay (Ctrl+F), which tells you how much VRAM you have and how much is used.
A more accurate indication is given by a free tool,
GPU-Z, which tells you exactly how much VRAM is in use, as some may be used by other programs. So it is best to just run iClone and nothing else.
I think your card has 8GB VRAM. I have a 1080Ti with 11GB and I also run into limits.
It's a bit of work but the best cause of action is to optimize texture use. So when I export from CC, I use 2K for the face and maybe the upper clothing and then less for the rest. Anything in the background can have lower textures too.
Also, in case you use Substance materials, it is best to bake these to regular textures.
iClone will also use regular RAM, so it is good to check with Task Manager how much is used.
Also, check the location of your iClone Temp directory. This would be on the C: drive by default. If you say there is little space left, then that may be another bottleneck. You can change the location of the Temp directory in the Preference settings.