You might consider buying a cheap 7" tablet on Amazon and using that for your dji. Assuming your controller can handle the size. I have an older dji and that's what I do. Nothing of importance on the tablet. I think a Lenovo 7" was like $99 when I got it.

Regardless of company size or popularity, that is sketchy. While not perfect by any means , google play affords some security scanning and privacy awareness info on apps that are published there. Others are probably right. And I bet DJI probably wanted to skip that step to trade for ease of management. But any time you install something off a non play store repo you take on a little extra risk.

I've owned two dji products, the Air2S and osmo... 3? Both needed apps downloaded not via Google Play.

Given the size and reputation of DJI, you shouldn't have any concerns.

From memory there's a fee in order to publish an app the the play store so I would assume they're circumventing that by hosting the app on their own website. You could also upload the app to virustotal and see what that says if you are concerned.