The monsters get harder mostly by doing more damage and having more hp (once you’re at a certain level, elite packs can spawn with 4 affixes but that’s the top in terms of complexity of opponents). Since the levels are timed, being able to take down big groups of monsters or various types of elite packs quickly and without dying is essential to completing greater rifts. It’s a much more exciting experience than running a normal rift where you are just killing stuff and get drops (and since you need to earn another try, it’s something that you care more about being good at).
I’m not really sure what you mean here in any sense more specific than that you don’t find it fun and think there is no possible build for which it would be fun. To put this another way, what would D3 have to be doing for you not to have this opinion of it? There are varied enemies that require you to adjust your approach, there are varied terrains that present problems or opportunities depending on what enemies you are facing, and there are a variety of ways your character can function that make it into a drastically different approach to things like skill timing, resource management, and space/crowd management. Most of these things seem irrelevant or barely present at low difficulty levels because you can just ignore them and bull your way through, then chalk up the occasional problem to an unlucky situation. At higher difficulty levels, though, you need to have a build that can deal with the things you run into and you need to play well. The game opens up significantly just past the point at which you stopped, so my main argument is not necessarily that you will enjoy it, but that your conclusion that you had seen all there was to see was not correct.
I agree that a good pure strategy title has more strategic considerations you need to deal with, and that a pure action title (like an FPS) has more “skill” during the action (though a lot of the skill is learning the maps and being good at aiming). D3, though, is a blend of those things - it has a real solid strategy layer in the character building side (which as I described a few pages ago has a surprising amount of depth to it), and it has a solid action game in the moment-to-moment play (where the skill comes more from managing your resources and your character’s location). These two things combine for an interesting and unique experience. It’s ultimately most similar to a CCG - you spend time crafting and tweaking your deck and acquiring the cards for it, and then you use the deck to play the game. It’s just that the card acquisition feels more organic and has more of that Skinner box / slot machine reward loop, while the “play” portion is obviously more dynamic. The part it’s missing is that you aren’t playing against an opponent, you are playing against a level, arcade-style. That can still be pretty entertaining, though, and the greater rifts show off its potential.
So I agree that the character building is the main thing and the fundamental gameplay is more arcade-y, but I don’t agree that it is as mindless as you make it out to be. I also think the character building is a deep strategic game that is interesting to play now that the player can control it. It’s no longer “kill stuff until it drops something better than you have, then kill more stuff or harder stuff because your numbers are larger.” Now you need to make decisions about when to sacrifice defense for offense, what kind of defense or offense you need most, which pieces of equipment are the most important to what you’d like to do, and both which skills will take the best advantage of what you already have and which skills would you like to be using and what equipment do you need to be able to switch. Do you run rifts to gather shards to gamble for a specific helm? Do you run bounties to try to get a certain cache drop? Do you just need money to upgrade gems or mats to do crafting / re-rolls? Even if we assumed that you had infinite time to play, there would be interesting considerations about what exactly is the best build for you, but the process of getting there is a lot of the fun.