Cream soda is probably a really good one to start with honestly. If you get that base dialed in, I’d imagine it makes moving into other flavors a lot easier.
I’d probably do small test batches and keep notes on each one so you can tweak one thing at a time without burning through a whole keg. Vanilla, sweetness, acidity and carbonation can all change how the final flavor comes across quite a bit.
The oils/extracts staying mixed evenly would be something I’d keep an eye on too, especially once it’s sitting in the keg for a while.
Sounds like a fun project though. Once you get the cream soda where you want it, you’ll probably have a pretty solid process built for everything after that.