I see where your coming from. But TBH the difference between those chips for what you want to do is minimal.
All three are excellent CPU's. No mistaking that. There are very few games, if any that would actually max out 8 cores 100%. Even games like BF V which is CPU heavy uses, 4-6 cores, and maybe a few threads. But none would be maxed out completely. Same with the 8c/8t 9700k or the 8c/16t 9900k.
I think you're focusing too much on numbers and HT. You have 6c/12t 8700k, 8c/8t 9700k and an 8c/16t 9900k. They all perform within just a few% of each other. If money was no object, then get 9900k and be happy out for the 4-5 years with some GPU upgrades in between to keep things steady. The 9700k will still be good for nearly the same amount of time. But equally so the 8700k wold have served well too.
Edit: Wildcard mentioned something earlier, about a side grade etc. That's exactly the issue here. The general rule of thumb for CPU/GPU upgrades, is to go new a min 2 tiers above where you are. So for CPU I7 8700k to 10700k or whatever. GPU GTX 960 to GTX 2060. Going from one gen to the next, specially with Intel means on average a 5% increase in performance over the previous gen. It's not worth the cost.