DM advice - how to gently nudge my players into optimization direction?

So, I'm a novice DM, took over for our group's previous DM who, for chronic health issues, can't run a game anymore but he can play as it's less taxing on him.

We're playing Tomb of Annihilation by player request, and at session zero we explicitly discussed:

• this is flavored as a meat grinder, and your characters will either git gud or die.
• I honestly expect each player to probably be on their 3rd character by the time the campaign finishes.
• while there's always some roleplay, the game emphasis will be much more on combat and exploration.
• If we manage to complete the campaign, I'm commissioning a drawing or painting of the victorious surviving group, standing atop the pile of corpses of their dead characters.

The group just hit level 5. I started them at level 3.

Here's where I'm running into difficulty:

• their one accidentally strong character is very much 80% of the reason they've survived the game thus far.
• They're very much doing doing character design for flavor and not for survival.
• I tried nudging them in the direction of optimization after they nearly TPK'd facing a random table encounter that, by rights, they should've had an easy time facing. There was immediate and unanimous pushback from the whole table. They all consider mechanical optimization cheesy and feel that it breaks immersion.
• Even pointing out "you have a cleric and a druid. both of you have the guidance cantrip. I don't think you've ever used it. It's free, you know - we can just assume you're concentrating on it any time unless stated otherwise. That was also immediately dismissed as metagaming.

If they'd had guidance going at the last combat encounter, they wouldn't have been caught by surprise and nearly TPK'd.

Anyway...I don't want to make the game not fun for them, but at the same time, this is a meatgrinder, and they know that I've explicitly stated that their mission is to survive where many, many others have failed.

Does anyone have advice on how to move the players in that direction?

One idea I had was to homebrew enemy stat blocks to essentially "try on" strong character builds I've been curious about for some time. I've heard that D&D basically breaks if you try to make it PVP - there's zero balance that way. But I wonder if a party of 4-5 level 5 characters, facing a single optimized level 5 "PC" enemy, getting nearly annihilated, might get the ball rolling.

What do you all think?