should have made a better title or clarified a bit better. Because obviously choosing a mechanical drive is dumb imo, but at the same time 2TB+ (SSD) is getting expensive. So yes, having an idea of what would make sense kinda has to be balanced with price.
4TB SSD as a minimum, so £400 minimum presently to add to a system, and that is SATA, if you want NVMe you are talking £700-900, unless you buy 4x 1TB and pop them in a 16x PCI-E adapter card, and run them in a PCI-E slot, still gonna be ~£500, and take up 16 PCI-E lanes.
I doubt that is practical for a great deal of people who only use their games for PC's unless they are very serious about it. A 1TB drive, with 16TB of storage would cost less, and you can just copy/paste games between the drives as you need them, would take about 9 minutes to copy a 100GB game from a modern large HDD to an SSD, and copying the other way makes no difference how long it takes, all you need to do is make sure you keep some space free on the SSD.