Endgame is a difficulty dial, not a checklist
The campaign gives you a build. Endgame asks you to keep that build alive while you turn the difficulty up.
The dial is the map item. Its tier and its modifiers decide how hard the area is, and you control both. Learning to set that dial correctly is most of endgame.
Order of operations
Do these in order. Skipping ahead is what makes endgame feel impossible.
1. Finish ascendancy. The trial content feeds Ascension, and ascendancy points are the largest upgrade your build gets. Do this before anything else.
2. Cap resistances. The cap is the same as it was in the campaign, and being under it hurts far more at higher tiers.
3. Replace weak gear slots. By now you know which slot is holding you back. Replace that one, not everything.
4. Upgrade the map item. Only once the build is settled does turning up the difficulty make sense.
Waystones
A waystone opens an area and carries its own modifiers. Those modifiers can make an area much harder than its tier suggests.
Read the modifiers before you run it. A waystone that adds damage and reduces your resistance is not a small step up.
Reroll or upgrade waystones rather than hoarding them. A stash full of waystones you are afraid to run is a stash doing nothing.
Choosing what to run
Run the tier you clear quickly, not the highest tier you can survive.
Speed beats difficulty. Two fast tiers produce more drops per hour than one slow tier that nearly kills you.
The trial content matters more here
Ascension 6 and 8 ask for level 60 and 80, plus multiple completions of the harder trial content. That is an endgame project, not something you force through with an unfinished build.
Get the earlier steps, settle the build, then go back.
Knowing when to stop pushing
If a tier takes more than one death per map, drop back a tier. The goal is a tier where you are comfortable, because that is where the currency per hour is highest.
Endgame progress is measured in drops per hour, not in the highest tier you touched once.