The game i propose fits non of your described demands but still i need to recommend it being the last game i am left to still play even after years.

Nethack.

On N900 you can play locally on the precompiled Unnethack variant but it is pretty dated.
My recommendation would be to start with the vanilla version before testing any variant since development is still active (3.7 released today!) and the Wiki is most detailed.

Easiest approach is to connect to alt.org via telnet from any terminal and explore stuff from there on.

An even better curated server is hardfought.org.
They offer to play any variant and always the most updated vanilla version via ssh.

The game has the steepest learning curve you can imagine and uses most possible combination of your standard Keyboard to control.
So any Hardware Keyboard Phone is a perfect fit for mobile nethack fun.

Your character is a simple @ symbol which you steer around the random generated ascii dungeons via "hjkl" and "yubn" for diagonal steps.
Your ultimate goal is to retrieve the Amulet of Yendor from the up to 52 levels deep dungeon and escape with it to the level -5 heavenly Astral Plane.
There you offer it to your deity and thus ascend to Demigood hood, winning the Game.

Since it is was created in 1984 there are now many perfect players with awesome skills winning the game in less than two hours realtime.
But i can not imagine any human brain mastering nethack unspoiled. It is just too complex.
Playing intensly in 2017, i managed to ascend my first game after years of casually failing. Recently had my 7th ascencion only.

Why should you even start this waste of time?
For me it is continuity and simplicity.

Since any game is a waste of time anyway, why not try one where you have long lasting stats on a public server where you can see long run improvements and can pick up your save game from any machine with a terminal?
My wife had countless mini games on her phone with meaningless achievements and short term pleasure.
Every time i fail, i learn for the next game

Speaking of fail, nethack is a "Perma-Death" turn based Dungeon and Dragons themed "Rogue like" type of game.
Rogue being the name of the first game with attributes like:
You can save and restore a game anytime and like chess, you have all the time in the world to plan every single turn before you execute it.
But if you die, you are dead and the game is lost.

The saying is, most games want you to win for you to have short live fun (financial exploitation is quite easy), Nethack only wants your death.

Writing this lines i again realize how much i love nethack and if you have any question, please ask.
I am more than happy to help.

For starters here is my fleshed out config for public servers for you to copy and have some nice IBM 850 character set eyecandy and the ncurses interface with always visible inventory and last messages.
For N900 a different config might be needed but this one scales perfectly on Pro1 and 2K Laptop.

(y/z is adapted to qwertz in above config, for qwerty change OPTIONS=number_pad:-1 to 0)