Second, any details given will inevitably devolve into utterly unhelpful nitpicking about those specific builds rather than the continuing general problem. Missing the forest for the trees.
This is not a reasonable position for you to hold re: 5E, which is why you aren't getting the feedback you want.
This is 5th edition. This is not 4th. This is very definitely not 3rd. The disparity between a sane character's performance (which you yourself outlined in the original post) and an extremely-optimized one, in most cases, is typically like 20% DPR or or 15% hit chance or 10-15% HP. Especially at lower levels. Most of the ridiculously optimized builds that people post about don't even "come online" until level 8 or 10 or even 15 or more. None of this is the game-breaking level of disruption that you're describing, nor does it even approach it.
In 5E, there are a handful of things which can cause genuine problems, almost all of them the result of multiclassing. You are wrong to complain about "missing the forest for the trees". It is you who is incorrectly seeing a "forest" when there is in fact a small stand of trees, and you are basically saying it's the Amazon Rain Forest because, one presumes, you're used to games like 3E were there absolutely was an Amazon Rain Forest of overpowered builds, thanks to PrCs, Monkey Grip, Chain whips, a bazillion feats, LAs and so on.
Players that go out of their way to break the game is the problem.
I get that you're saying you don't want your players to find out, but we need to know what it is that you think is "breaking the game".
Why?
Because different people have wildly different standards. You've given a standard for what you consider perfectly reasonable, for what you're not talking about, but this still leaves and absolutely vast arena within which we could be discussing the issues. For some people, mildly optimized characters like, say, a Warlock who actually understands how Warlock mechanics work is "breaking the game". I've literally seen the complaint that a Warlock who had Agonizing Blast and and used Hex correctly was "breaking the game". On the other hand, some people are dealing with pretty powerful Paladin/Sorcerer and Warlock/Paladin and similar combos, and saying they "break the game" - which is still not really true but a lot closer. On the gripping hand, some people are dealing with stuff that's actually entirely exploit-oriented, not really following the rules dodgy stuff enabled solely by dubious/creative interpretations (which actually require DM buy-in, note) of various abilities/spells/etc.
The odds are extremely high than 90% of the problems you're concerned about are solved by "ban multiclassing, it's optional anyway".
Really the only way this isn't true is if the characters are over level 10, and have been thus working on exotic builds for a lot of levels, and those builds are now "kicking in". But I'm guessing that isn't the case. Even then, there are a relatively small number of spells, Feats and class features, almost all of them optional, which can potentially cause genuinely serious problems.
It's also possible you're describing perfectly normal play that doesn't cause issues for most DMs to be "breaking the game". I don't even say that to judge you or be mean. I've played with DMs like that. One of my friends is like that. He is just completely unable to deal with efficiently built characters, like, psychologically, when he DMs (we thus don't have him DM D&D anymore). He himself is not a good optimizer (a great tactician but...) and continually and rather inexplicably underestimates PC abilities, and does stuff like make main villains be normal NPCs (not Legendary or even just high-HP or the like), then get really confused and upset when they get splattered just like, well, other normal NPCs. But I'm assuming it's worse than that.
Anyway we need examples. If your players are here, they're probably bored of this thread by now anyway. And again you are wrong to think specifics don't solve the problem, because in 5E, they can. Not in all systems, but in 5E, yes.
(As an aside, I am a hardened veteran of the 1990s "munchkin wars", and have played with people who genuinely like breaking the game and didn't even care if others had fun and so on, so it's not like I'm pretending these guys don't exist. But standards vary wildly for what "breaking the game" is, and different RPGs have entirely different solutions. With 3.XE, you really genuinely need players to just agree not to do it. But with 5E, that's not the case.)