Seriously, just about everything I’ve tried is fundamentally a Pay-to-Win option, not a lot of help.
It isn’t really a strategy game precisely, but for a fun, lightweight game with planning elements, I’d recommend Fallout Shelter. Especially if you’re a Fallout fan of course. It’s more about base building, resource management, and collecting geegaws, but while you can pay for very minor geekups, I never needed to bother. It’s a great timewaster albeit with lots of downtime while you’re waiting for people to come/go, train, or reproduce.
Almost anything resembling a fun RTS for me was full of “watch this 30 second video” every 3-5 minutes or a microtransaction hell for pay-to-win or wait 24 hours for the next step with absolutely nothing to do in the meantime.
Having said that, my friends that are more phone-gaming rather than PC gaming (my vice) suggested the following:
Super 90s RTS style game, no sub or microtransactions, one time purchase of $1.99
I mean, it’s freaking X-Com, popular with my friends and myself in PC format. Again, no microtransactions or subs, but $14.99 one time purchase. My friend swears it’s a faithful (including the lethality) translation of the PC game, and I mostly trust them, but have zero personal experience.
Lastly, for Warhammer 40K fans:
Now, this I played on the PC. And I was… underwhelmed. There it felt like an Android/iPhone game scaled up to the PC, with utterly insufficient content. Especially at the price point they asked at the time I bought it (even on sale, at the time it was $19.99 for 50% off). But it should work well IMHO as a tablet game. I’d hesitate on the average phone, because you don’t want to try fiddly movement controls on a small touchscreen for this style of RTS/Shooter, but should work well on a 10inch tablet.
And it’s $9.99 - with no other paid features.
Still, as always, double check the android store to make sure it’s compatible with your tablet and current OS. 4 years is pretty long in the market, and of course, some tablets are more compatible than others.