First, you need to get together as a group, outside the session, and decide what game you're playing. Should combat be deadly? Should characters be optimized? You need to agree on that. It's possible to rotate between everyone's favorite activities to an extent, but only if the fundamentals are clear. It sounds like some of the PC's don't enjoy near-death experiences, and that's okay. Remember, you're there to have fun, and it's not fun unless it's fun for everyone.
An analogy: supposing several of you had agreed to get together and play cards every week. Now several of you really like Hearts, and one prefers Go Fish. But every week, one player grabs the deck and announces "OK, let's play some poker. Ante up, everybody." What would you do? Hopefully after a couple times, you'd speak up, right? It sounds like you know you need to say something, but I want to illustrate that this is a disconnect that (from what I can see) affects the entire group, and the entire group needs to address it together.
Ideally, you could do this not during your regular time, so you're neither saying something that might invalidate what the DM has planned for the next three hours, nor coming off another session where people are frustrated and tired. If you can get people together at a different time to say "Hey, let's review our expectations for the game", I'd do so. Or maybe suggest this in lieu of playing one week. If others recognize that there's an issue, they should be on board.
You've indicated that you're not sure what to say, so here's some sample dialogue for you: "Joe, it seems like there's a disconnect in what we're each trying to get out of this game. It seems you really want encounters to be challenging, even deadly. But for me, when I build a powerful fighting character, I really want them to feel powerful most of the time, able to easily defeat routine enemies and stand on an equal footing with strong ones."
Then let all the other PC's and the DM weigh in.
Be prepared for this to go differently than you expect. Maybe others will say they enjoy these deadly fights where you get out by the skin of your teeth. Maybe one of the casters will say it is frustrating when it seems like a fight between the martial characters and the DM, and they're on the sidelines. The key is to listen to what each person wants, even if it doesn't match what you want or what anyone else wants. Remember, there are many different ways to play DND, and none of them are objectively better or worse - what matters is having fun or not.
Then say, "Ok, it sounds like we have some things in common [if that's true] and there are some things we see differently. I found this checklist online - would you be willing to go through it together and see if there's a style that's at least acceptable for everyone?" (Remember, this is about consensus - do not fill it out separately and then "vote". You want everyone to say "yeah" or at least "we could try that, I guess" for every choice.)
I'm not gonna lie to you - I'm not sure there is a style that all of you will enjoy. You may find that, having discussed and acknowledged the gap, you'll be willing to compromise and try going along with the majority in order to make the game happen. As long as everyone is sincere about being willing to give it a go, that can work. But you also may find that one or more of you wants something totally different from the rest, to the point where it doesn't make sense to play together. (Analogy time again: if you really don't like Monopoly, you're unlikely to sign up for many hours a month of it just for the sake of staying in a board game group.) That's fine, and not playing because you have different tastes in games is not a referendum on anyone personally.
There are questions of party balance and encounter design lurking in here. But I've removed specific suggestions about that because the bigger issue really needs to be resolved first. When you or your DM come back to us and say "OK, we all want the same thing, but we're not sure how to achieve it" (probably in a new question), we'll be here.