The API Issue
Listen, Open-source and free should remain free. So should the API usage therein. Only 2 options for API are given, both with a(n) (eventual) pricetag. I get that you guys probably get that sweet-sweet commission, the no down-time guarantee, and "scaling". This also creates an unfair market for small, open projects like OpenStreetMap, which I really want to use instead of big tech or cloud computers. Your open-source freedom isn't free if you're required to pay. Thanks for your time.
Thanks for the feedback! I do want to set the record straight, the only compensation I have ever received for this project has been through the Donate button on GitHub (which amounts to less than $1/hour, not taking into account server costs for hosting free placefiles). Mapbox nor MapTiler provide any sort of kickback for this work. Mapbox and MapTiler both have free tiers that are more than plenty for Supercell Wx usage, with the latter not requiring a credit card. If that were to ever change, I would be scrambling to get a patch in for some other mapping source.
I do want to provide some sort of offline map capability, but Mapbox and MapTiler have made things easy and free, while allowing me to focus on other capabilities in the software.
I could see the third option being OpenStreetMap, which would then download the files to your PC, and serve map tiles to the library. However, this isn't trivial, and would require time away from other features. I'm open to contributions as well.
In the mean time, if you stand up your own tile server with a style.json file, you could put that in Custom Map URL, and it would probably just work.