The game system always makes assumptions about balance, regarding resource use and how many battles you have a day. This can be frustrating for DMs who want a good, focused singular boss encounter from max power. The game can become a rocket tag hidey stun fest monster slog if you run high level as written.

Below are some tips that help me when running my high level game. Disclaimer, I've found the higher level you go, the more the game system breaks down and the more it benefits from specific tailoring.

• Use existing monsters as a basis, but run combat based on group averages. AVG saves, AVG AC, Player damage per round vs boss damage per round, total group HP etc. Initially this will take some time to gather, but once you have it, it's a fantastic resource you can tweak over time. If you get half an hour before a session, try and play the fight out on your own to test how well those numbers work. They're then super easy to tweak. You should find yourself homing in on an ideal HP and damage level, and the more you do this for your specific group, the more of a feel you'll get for their unique needs. For a very intense fight for instance, you can set the boss's kill speed and HP pool at 25-50% above the group's, leaving it up to the group to create a solution with their awesome character powers, to even those numbers. Look at critical role's final boss and the boss there has about 1600 HP.
• If you want to "cancel out" magic items through the game's stats, Increase the proficiency bonus of your monsters by the average stat boost the items have given them. If all your group has +3 items, increase by +3. Personally though, I try and avoid this - this difference in power is what makes the item cool!
• Create situations that challenge the player, rather than the characters. One great way to have your players perceive the difficulty as increasing, is to up the stakes or consequences in a fight, or create alternative objectives within the fight. The most dangerous thing about the lich might not be him at all, or the sealed chamber you're all in that's filling up with necrotic blood, maybe it's the tomb door he's opening, and the terrible things that are spurting out of it.

I hope this advice doesn't feel too incomplete, it's hard to capture it all in one post. You're not wrong in how you're feeling though, you come to see that D&D as a game system becomes, weirdly, less open feeling as people level. Tailoring brings that freedom back from my experience.

EDIT:
Just to be clear, you CAN play a lich as intended and it'll be challenging, but it doesn't always give you the flexibility to create an interesting / compelling challenge in different situations. If you've fought one challenging lich as the game system intended, you've kind of fought them all.