I probably need to upgrade my PC.
It seems that a bunch of people try to get hold of 9070 XT cards these days, in which case old cards might come up for sale. Neuro-sama ran quite well on a 1080 TI (upgraded away from it around December 2023 I think). Sure the card is old in terms of FPS, but the 11 GB of VRAM makes it a decent low cost card for AI if the AI in question can run on a 1000 series card. Apparently Nvidia has said something about dropping support for the 1000 series in their driver updates, so that's something to consider, but it will likely also affect price.
I can't tell if it's a good option for you since I don't know your budget, what you use it for etc, but if it's only for AI voices, then new cards will be multiple times more expensive if you want to get something, which doesn't have less VRAM. Obviously when considering used vs new, there is also warranty and similar stuff to consider.
In case you aren't aware, Nvidia trusted a fake rumor that AMD was releasing some GPU, which would beat Nvidia, so Nvidia upgraded the 1080 to a 1080 TI where they overclocked it and gave it 3 GB more VRAM while still selling at a somewhat affordable price (particularly compared to modern prices). AMD didn't release anything to compete against that and Nvidia got stuck competing against their own old card because they dropped down VRAM again and the next card, which didn't have less VRAM was the 3090 3 years later, which started out at twice the price of the 1080 TI. That's why the 1080 TI can be interesting if your goal is getting enough VRAM for AI on a low budget.
Also, very important to me, I don't want to rub off anyone or any community the wrong way. Is there any rules that I should follow? Is there any forbidden things that I should be aware of?
Probably not unless you start to do something like claiming you are the creator of The Headmaster, leak Patreon only releases or something like that. As long as you work on the free version from Itch.com (linked in first post) and declare it to be a third party addon, then I don't see why anybody would get upset. You may consider joining the discord channel (also first post) as that is the main approach to get in touch with the development. They do look at this thread from time to time, but less frequently than discord. Then again a lot of people only look at the forum, so abandoning the forum is also not the answer.