I've put mine on Google Play and Amazon. There are a few different hoops to jump through with Amazon (but once setup you can just install the same apk I think) and I seem to remember 'only' getting about 10% of the install user base on Amazon as I did on Google Play but your stuff might fare a bit better.
I wouldn't have thought that the Samsung store is heavily used. Huawei might be a good call, especially if those devices no longer come with access to the Play Store.
For Google Play I used to have 2 Apps, free (ad supported) and paid for.
I could deploy the free App to Amazon straight. The paid for one, is a little different - I think I removed the pay mechanism and then applied the Amazon protection to the APK so that when Users paid for it in Amazon they couldn't install it elsewhere etc..
So for 4 'points of sale' to those two stores I had 3 versions of the App.
A 10% uplift is probably a good return. Although I wish I'd put a different advertising id in the Amazon free App so that I could discern ROI between the two in Admob.
To be honest I'm almost considering making my current Apps free and then including a 'pay me what you think' button. Mine generate very little income at the moment, although they are motorsport related and either there haven't been the events or I just haven't bothered to update them as things were changing on a rapid basis.
