The main endgame, Monolith of Fate where you run echos, is very easy when you complete the story. I’ve usually been under-levelled by 10+. They path off in different directions and have different rewards types (e.g. gloves) so you can target gear that you most need. In this respect it’s a much better system than PoE as there’s far less layers of rng. You also don’t have to worry about map sustain, the echos don’t cost any type of resource.
Once you’ve completed the level 90 timelines you start farming empowered echos and have control over the difficulty based on how far from the starting point you go. The main issue is that this gets pretty monotonous having gone through so many echos already to get to that point.
There are also dungeons which require keys. These are accessible much earlier on the game and you unlock new difficulty levels by completing it each time. Lowest level is 20 but keys are relatively rare at that stage.