One thing I feel is important for you to know is that it is very unlikely for you to find the portraits you need.
I never tried my hand at making pixel art, but I know these take time and effort to be made, and it’s unlikely people will make them for free, specially since as you said it is still an idea, so there’s no guarantee that the project won’t be dropped later on, which would make all the effort put into them be somewhat wasted.
The fact we even have the repo that is so well organized and extensive is a miracle by itself honestly.
You will either need to work around the limits of what the repo has, learn how to make portraits or comission someone to do them for you.
Edit: Matthieu’s CCC generally are a good way to find portraits for characters: CCC - Princess & Mercenary Duo - Dieck, Guinivere, Saber, Celica (September 2024)
There is also this post, maybe it can help: Is there more resources besides the repository?