I love this sort of technical discussion. My team and I talked about this very issue in a concept we were working on a few months back. Though we scrapped the idea, we realized that it might not be immediately obvious to devs looking to create seed-based games that every random system needs its own sort of seed created the same way every time, and it's awesome that you're discussing it! We were going to go with the alternate solution you though of though, creating one for each RNG system when the main RNG is initialized. But, yours seems much easier to scale as new systems are added, so I'll keep that in the back of my mind next time the issue comes up!
Viewing post in Brewers of Khazad-Dun: Devlog 02 Better RNG comments
Thanks for reading! Yeah it's the sort of problem you don't really think about until it's staring you in the face.
It's always re-assuring to hear other devs coming up with the same/similar solutions as me. Makes me feel like I'm not crazy lol. If you do end up using either solution and scaling it, let me know!