This seems like a possible contender. I’m currently giving it a test now just to see what it’s like. I’m fairly new to all this, what do you mean by their geo-stack not being open?
Edit: I’m not sure this one will be able to work after all. I think it probably needs to be mobile focused, and this one seems to want to be used on desktop. I tried uploading a .pdf for a georeferenced map and it just got stuck in the loading process (this could be just because my internet is shit right now). I may give it another try, but for now I might have to keep looking.