I agree that the game quickly tapers off once you get the gear pieces you need. Trying to go for "better" versions of the same gear gets old fast when you realize how much it takes to get it.
I finished Tower 100. The only thing I am trying to do now is get the final piece needed for the season event that has you taking on...everything.
I got one Mythic Trickster key so far. 300 sounds about right for how many keys you have to go through to find one.
I was going to try for 300 but I hate over grinding for something that comes with very little of anything. Once you get all of the special Paragon slots, having just stats to add gets boring.