I've recently started looking into this mod's overlays to better match my UI from what I have on other MMO's and found the custom overlay functionality to be quite neat.

However, I can't really find much documentation on how I should write out the template strings for both Text and Value templates apart from the %% strings on wynntils.com/faq which didn't work until I tried stuff like taking out the percentages and/or using curly brackets instead.

Have I missed some detail or is it a matter of not enough documentation around them?
Is there more functionality to them (like formatting numbers akin to a %02d format string) that isn't written?

It would be nice to have an example in the descriptions for these fields or in general some better documentation.

Are there current efforts towards this I can maybe contribute to?

The Wiki on Github and our Discord has a lot of info regarding Functions, which are the "template strings". Please check them out, I think they should answer your questions.

Thanks, I'll give those a look 😄