I mean, is it okay to start with UWP apps? I ask this because (a) I'm confused regarding what Microsoft's plans are and (b) I don't really know if UWP will be there in the future, given the plans regarding Andromeda, Windows 10 S and others. I just need a relevant starting point. Any help/suggestion would be appreciated!
My advice, one dev to another, is target iOS first, then Microsoft, then Android.
Why iOS? Because Apple supports their platform with really great devices like the iPhone and iPad. And frankly because Apple customers tend to spend money. Most app developers actually making a living today are doing so on the Apple Store.
As for Microsoft, I'd say put it 2nd to iOS IF, and it's a BIG IF, if you aren't depending on "mobile" customers. Nadella and Microsoft management have clearly abandoned everything mobile. And no, the new 10 inch Surface Go doesn't count. Without a pocketable device, aka a
phone, there is no "universal" in Universal Windows Platform. So only if your app targets mainly laptop owner or laptop-like device owners (ie Surface Pro, Surface Go), then target the UWP platform 2nd. If not, then skip UWP altogether and move on to Android.
That said... Android users are mostly all a bunch of hippies who believe everything in life should be free. Google promotes this theology by charging nothing for their services, even expensive services like Google Photos, cashing in via ADVERTISING instead, like the self-righteous hypocrites they are.
So iOS first (regarless of what form factor you are targeting), UWP second ONLY if you don't depend on mobile customers, and Android 3rd, but make sure you don't charge for your app on the Play store, give it away for free and load it up with ads instead! It's the only thing those hippies understand.