As I said in #1149805806368333895, I would recommend against doing this. I think you should get yourself a good main PC and run Salad on it when you're not gaming or otherwise intensively using it.
I'm just going to be blunt - You're thinking about this wrong, and need to rethink this.
The 4090 isn't "exclusively for 4k" - You can use it perfectly fine with a 1080p display. If you still don't want to believe that, according to your logic, a 7900XTX would also be overkill for 1080p.
Some quick points:
- SLI is dead. Don't bother running multiple GPUs in one PC.
- HDDs are dead. Don't bother wasting multiple collective days of your time for things to load.
- LGA 1151v2 is dead. That platform was released 5 years ago and is far beyond being EOL now.
- If you're restrained for physical space, just get one PC.