The other stores are much smaller, but every little helps...
Admob works on Amazon.

I upload the same App to all 3 stores (if free and with Admob). Of course they don't come pre-installed with Google Play services, maps etc.. So that needs to be handled.
You don't necessarily need a Kindle Fire - it's not just Amazon devices that access that store. Although I did buy one a few years ago as my Apps kept on getting rejected - I couldn't work out why until I had one to play with and could see the issues.
They are quite good at auto-testing the Apps for you as well and provide feedback where necessary. Up until recently my Apps were approved really quickly but then the change in Maps services meant they got rejected for the Amazon devices - now updated again.
I also produce a Paid For App in Amazon - no licensing server check (obviously) but get them to apply their own DRM.
All in all it is a smaller store, but seems to provide maybe an extra tenth on top of Google Play. I should have split out my Admob reporting so that it gives a different id so that I could see the impact of that as well, but didn't - so can't really tell the impact of Amazon free downloads on my App revenue.
You get quite different feedback from Amazon Store Users which is useful.
I think that your sort of Apps (i.e. games) may well do quite well on Amazon - especially Amazon Underground - which might be interesting for you to try.