In a way, W40K is ideal for mobile games. You already have some fabulous character designs that are distinct and easily recognizable. And they all speak in cliche phrases anyway, so nobody will notice your lack of writing skills. Speaking of which, your story can be “space marines are fighting xenos because crashed and artifact”, and everyone would be perfectly happy.
The gameplay is rather solid: hexes, different terraint types, one move one action per turn, one unique skill per character that can be used once per battle. Where it all breaks, of course, is the monetisation and the “numbers game”. No matter how good a tactian you are, you won’t be able to progress if your units are damaging enemies for 1HP. To level them, you need not only EXP, but also coins, which you get by completing missions. And to do missions there is another resource, which I don’t even care what it’s called, that recovers with real time, but also can be bought with very real money. Then the new characters are farmes through a gacha system.