How to deal with this GM?

[Reversefigure4]

Well, it sounds like you -do- want it on easy mode, relative to this GM's level of "normal" difficulty. Just like different video games have different relative settings (Dark Soul's easy mode is more equivalent to Kirby on Nightmare Mode, not the same as Kirby on easy, even though both games are set to "easy").

Try asking for easy mode, and if you get it, see how do you like it compared to the gm's "normal"? It's no judgement on you - if your GM's Normal difficulty is "Boss kills half the party, guaranteed, in the first action", then you probably want Super Easy difficulty, where the Boss merely hits for good damage and takes several rounds to take down.

[TinyMushroom]

Sadly, that's just... not what 5e is good at. Have you guys tried the "gritty realism" variant, or some other sort of adjustment to the rest structure?

We haven't tried that yet, if you have any suggestions I'd love to hear it. I think an adjustment to the rest structure could help, but most of all we need a way to extract from encounters. Gritty Realism sounds like it punishes combat even harder and stretches the downtime even longer, which we definitely don't want.

How is this sniper able to take shots at you while being totally immune to return fire?

Our darkvision is limited to 60 feet, and he's at least 300 feet away. We don't know how this character can see so far or afford to spend as much on poison and bombs as it is (that stuff doesn't come cheap at all...). Were definitely going to extract from that sidequest, since it's above what we can handle. It was just an example of how lethal situations can get in this campaign.

[King of Nowhere]

Ok, sure - and the result will be that the frontline PCs (at least) are dead. Is that a desirable result? I'm not saying it can't be, but it doesn't sound like it was a good thing in this case.

Because when you're the GM - the one deciding who the enemies are and what tactics they'll use - then you can't hide behind "it's what the character would do". If you put a character in who would logically TPK the group, and they do, then you, not the fictional character, have decided to TPK the group.

So maybe there shouldn't have been a "brutal tactically optimal fighter" there, or if so then there shouldn't have been whoever launched those fireballs.
.

Yes, the dm gave a too hard encounter. Now he has to intentionally gimp the npcs to let the players win.
I don't like that. It is a form of deus ex machina: you'd be toast, except that the enemies suddenly and inexplicably start acting stupid.
Much better to have weaker enemies and let the pcs feel like they actually earned the victory.

Alternatively, if the dm want hard fights, he should build the campaign with the premise that needing a fee resurrections every once in a while is expected and not a big deal

[King of Nowhere]

If the DM hasn't actually killed any players yet, maybe his intention is that every fight should be desperate and frightening, but he doesn't intend to ACTUALLY let you die. I'm thinking that could be what he's doing with that "Death Rolls in secret" houserule: he wants it to be nerve-wracking, but doesn't want you to actually die, so he wants the ability to fudge the death save. Have the villains had you at their mercy, then gloated and left without finishing you off?

I suspect this as well. Many dm try to do this, because it works well in movies. So they think, if they try to make the fight super hard, but won't actually kill the pcs, then it will be super exciting, overcoming impossible odds.
They don't realize it doesn't work. It only can work if they are dming for their little 5 yo brother. It can work for a bunch of middle graders, if they are particularly clueless.
People with a bit of perception will realize the dm is setting up impossible fights and letting the players win by deus ex machina. It removes agency from the players. And the players know it doesn't matter what they do, they will always end up losing and then be handed victory cheaply.

Those kind of dm often railroad hard too, because if you can fudge rolls and opponents to make a more "exciting" fight, then you can invalidate players choices to make a more "exciting" story.
They don't realize that the players are easily seeing through their paper-thin attempts at dissimulation, that the story is actually lame because it invalidates the players.
They may have a good world to go along with, and may actually be excellent storytellers. But they are using the wrong medium