I have my own reservations with regards to Google, but it is more with their T&C's - effectively forbidding you to use any non Google content, ie. including your own, with their maps. Cost is rarely a problem (the $200 monthly free credit is high bar to cross).
If your only problem is getting Open Source map tiles you might have been too quick with writing the ggmap package off - it is perfectly capable of producing maps based on OpenStreetMap basemaps (e.g. those prepared by Stamen co. - my favorite is toner, but I digress...).
Consider the example in this question: Error working with OpenStreetMap spatial Data
But you will need to geocode your data first. For geocoding consider the package OpenCage based on OpenCage API. You will need to register for the API, but again should have no problem with costs of a reasonable traffic (2,500 free API calls monthly).
Note that geocoding can be tricky - places called "Berlin" are typical example - and might require manual cleanup.