The "depth" of the strategy is also sooo much more than an FPS

You only think that because you know more about LoL. As someone very into Overwatch, it's an absurdly complicated game where you only know about the complexity once you learn about it. After having learned a lot more about the game, I can look at high-level play and "see" a lot more of the things they're doing due to the level of complexity of the game, just like you can see that same stuff watching LoL.

At a high level, Overwatch is complicated because it effectively combines MOBA elements with FPS elements. It's not really a straight FPS, and the higher skill you get the more you have to pay attention to the MOBA elements in order to do well. As an example: Zarya is a tank whose entire ability kit (sans ultimate) is built around using an ability on herself and a teammate (on different cooldowns) to protect them from damage/negative effects for 2 seconds at a time. This ability can be destroyed if 200 damage is done to the player it's affecting, and Zarya gains increased damage if damage is done to this ability up to a cap. It sounds simple, but players who have mastered Zarya are ones who know _when_ to use these two abilities, which order, etc. which has a lot to do with the current state of the game. Bad Zarya players use both abilities instantly and simultaneously the second they see the opponent. It's all about timing abilities... which sounds a lot like playing a MOBA. But you also have to be good at aim and master the two firing modes she has, among the many, many other things to pay attention to during gameplay.

I don't whether LoL or Overwatch has more "depth", but they both have a lot of it.