Loading games off of hard drives gives you perfectly acceptable loading times.

I remember I used to have a 4TB HGST UltraStar, it'd load GTA V in around 20 seconds, from clicking continue story to getting into the overworld. Sure, an SSD will do it in 5 seconds, but for the significant price difference between high capacity SSDs and high capacity hard drives, 15 seconds of my life is a tradeoff worth making. Especially in OP's case. 4TB SSD's are $350+, while 4TB hard drives can be had for $100.

I think you might be overestimating how slow hard drives are. They're not fast, but they're not so slow that they're unusable for playing games off of, launching applications from, and keeping some local files on.

The only thing that I don't really think hard drives are good for is booting off of. Leads to a sluggish overall desktop.