On the topic of “4X games playable on a mobile device” - the one I linked above is kinda shit, but I found a different one that is totally free and has way better depth: Birth of the Empires.
It’s more or less a direct rip of Birth of the Federation - but hey, it’s free and it’s on Android, so what the heck.
The main issues I’m seeing so far is that the UI is tiny on most devices so it can be challenging to click around, and the game is fairly obtuse and has no real tutorial to speak of. There’s a “?” icon on every screen that opens a help page for that screen, but there’s no coherent step-by-step tutorial. You’re mostly on your own.
This one has kept my attention so far because it seems like there’s meaningful depth here. We’ll see if it lasts.