POE 2 "One Shot" Bosses

This is my second time ever playing PoE. The first time I played in The Crucible league and had a decently good time but only ever did maps and no bosses. Relatively new to the ARPG genre only really starting to play with Diablo 4. When I first saw the trailer for PoE I was most excited for Pinnacle bosses but now that I have gotten to the end game my joy for them has been completely sucked out.
During the demo I saw the Pinnacle boss for the breach and decided when I got to the end game that that would be the first boss I went after. After doing my first breach in getting nine whole splinters I hovered over them and realized that I would need to do 30 more breaches in order to get enough splinters to attempt the boss. 30 more maps, that's almost 3 hours of farming. And then when I finally did get the breach stone I did some research and realized that you only get one attempt at the boss.
I understand the Pinnacle bosses are meant to be hard. I enjoy hard bosses but the difference between this and Elden ring is that when you're fighting a hard boss in Elden ring and you die, you spawn right in front of it so you can try again and learn from your mistakes.
When I realized this now I'm worried to even use my breech Stones because of the frustration I would feel if I ended up dying to one of the numerous one shot mechanics I saw in the video of the fight. Are you meant to just farm enough gear that you one shot the boss no questions asked if so what's the point of even having a cool fight if the only logical way to fight the boss is the one shot it lest you lose hours of work.
Is this just a new player gripe? Do more experienced players also feel this frustration with Pinnacle bosses? Honestly it's so disappointing because it was the thing I was looking forward to most And now I don't even want to attempt them because of the risk involved.

This end-game bosses thing problem also involves the costs, if the mats are worth like 5 div at current rates and you grind them just to go in and get one-shot instantly and can't go back that's an issue for sure, and that's what's happening now.
Being at a boss the first time you can't be expected to know to execute the mechanics perfectly or know how strong you must be to not get one-shot by it. Plus even the best players sometimes die from one ability and that crap is not normal when you got end-game gear and feel ready to go against the boss after the grind, getting the stone by yourself should mean you earned the right for a fair shot (not counting here bought / carried mats runs or whatever), not a short glimpse before being obliterated with no better knowledge / info for next time. And it's not like it's intuitive - you get no hint what to do whatsoever.
This makes it worse than souls-like and puts people in a HC situation that should not exist in a SC environment you opt for (you only got one life and you lose), more so as a new / learning player. The game was and should be difficult, but also fun - not frustrating!