How to deal with this GM?

[KorvinStarmast]

The second one might require at least an attempt to see things from his perspective. I can have some sympathy here - havening DM'd things that didn't go as planned.

The DM wants to the players to engage with the content he's created. Most DM's do.

Dialing the difficulty up to 11 might not be about crushing the party, but rather be about providing a specific incentive to play smart.

From the OP, it appears to me that the front liners are taking it in the face, and the back liners may not be effective at debuffing enemies.

Or it might be about a belief that an exciting game is one where characters can actually die (and the players beleive it), and this needs a bit of a demonstration.

Yes. Without the real prospect of character death some of the edge is taken off of the campaign. (And this usually needs to be discussed at session 0).

When the DM is giving you quests that are not level appropriate, how is he doing it? Is it a worldbuilding thing - i.e. explaining the famine by the presence of an ancient lich killing all the crops and the PCs jump up saying "what ho! a lich sounds a jolly good adventure for our level 3 characters! Hooray for now knowing scorching ray!". If they have people displaced and begging, is it part of painting a picture? Or is it a clear hook? Is the non-level appropriate encounter gently added to the world as a means to trick and stomp the party or is it a motivator and a yardstick to measure power development against?

Nicely presented, hope this helps the OP communicate with DM.

Were there warning signs that the encounter was not level appropriate?

This takes a bit of art to signal, and maybe the DM isn't great at it.

For the OP: you say the back liners are not getting hammered. I'd suggest a focus on debuffing the enemy might be where you can help your front liners more. Not sure what classes and spells you all have, but if you are blasters, not controllers, you will often leave your front liners hanging out to dry. Using spells to slow down, separate, and otherwise debuff the opponents is the best way to make your front liners more effectie. (Spells like slow, web, evard's tentacles, blindness, and so on).

Do you tend to have one really hard monster, or bunches? Spells like fear can break up groups of monsters, for example.