First and foremost, welcome to the LE Community.
Generally speaking, yes you will get alot “stronger/higher Level” then the Mobs you are facing, when you clear almost every Corner of the map, because mob kill exp is the majority of the exp you get(quest don’t really give much exp comapred to monster kills)
For the available “endgame” there are currently 2 different game modes available.
Monolith of Fate which is basically a endless chain of generated missions(kill the boss, kill all fire pillars, cleanse the forge etc.) with different modifiers(you can always pick and choose between 2 different modifiers that effect the enemies) e.g. enemies do more XY Type dmg, enemies all have an increased chance to dodge etc.
The further you chain this areas the longer those modifiers last, at first they are just for one run and then they start to last for 2, 3 or more runs and you have to choose a new modifier after every run so they modifiers start to overlap so you have multiple modifiers active at once.
Later down the line the modifiers get stronger also.
You dont have to make the chains in one session, each current lvl of Progress i saved on the character but if you die you start form 0.
Then you have a endless Arena Survival mode, every 5 waves you get a bunch of loot and can choose to get an endreward or contine 5 more waves, if you die you don’t get any endreward.
To enter the Arena you Need an Arena key, which Drops from the Monolith of Fate Content.(Every 5 Areas in MoF mode guaranteed)
That’s said, both of those endgame modes are scaled, so you cant outlevel them and they will always be appropiate for you character depending on how far you are going to go.