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You an purchase 'Aliens versus Humans' in the Play store which is a remake, but very close to the original. I think it is under $5.
EDIT: formatting
This kinda goes against your first requirement about being less brutal on you, but the original X-COM: UFO Defense (UFO: Enemy Unknown) can be played on Android phones with OpenXcom Extended Plus (OXCE+). You will need to own the original X-COM on GOG or Steam in order to copy over the game files to your phone/tablet. There's a bit of a learning curve involved, but as someone who started with the new XCOM series at Enemy Within, I've found UFO Defense to be just as fun once I got over that hump.
Original X-COM definitely has you covered with turn-based tactical strategy. Base building is there and is important because the layout you choose will directly affect your base defense missions. Character progression is there, but is not as personal as new XCOM; you carry more soldiers into battle and you can afford more of them, and thus they are more disposable. In fact, capturing certain aliens/tech while losing nearly every soldier on a mission can still be considered a success! I'm almost 6 months (game-time) into my current run and I've lost 48 soldiers out of 108 total hired. Graphics-wise it's obviously less intense than Enemy Within, and battery life shouldn't be affected as much. If you're interested, definitely look up guides on places like UFOpaedia and this strategy guide for an idea on how to learn the basics and play effectively. Here's a quick-start guide on Steam Community to get you started. Check out streamers on YouTube such as Meridian and GuavaMoment if you're more visual.
There's also an Android app in the Play Store called Aliens versus Humans that closely resembles X-COM but with a more touch-friendly UI. Haven't played it myself but it's $1.49 USD if you're interested.
I didn't play xcom ufo defence, but heard that "Aliens versus Humans" is pretty good clone of it. https://play.google.com/store/apps/details?id=ca.leisurerules.AvH
Aliens vs. Humans is a clone of the first XCom.
edit: oops someone else posted this while I was typing.