Great job, well done Andrew! And fantastic plots 
Regarding using the binomial for more than two candidates, I would avoid doing that, because that’s gonna underestimate the correlations between the candidates – i.e the zero sum game where one has to lose for another to win.
The model can’t take these correlations into account if the probabilities are estimated independently. A multinomial will be what you’re looking for here.
You can see how we did it for the 2022 French presidential elections here – associated podcast episode.
You’ll also see we’re using a custom GP in this model, instead of the random walk from the blog post you referenced. Today, I would very probably use an HSGP instead.
Hope this helps, and PyMCheers 