Again... relies entirely on assumptions.

Let's simplify this from an action economy perspetive so we aren't running into the reaction limits. Let’s assuming combat lasts four rounds, and on each of the first three rounds, you cast Polymorph (killer whale), and use your Chronal Shift to force a reroll if they save, generally with the goal of burning legendary resistances.

You have two massive assumptions there which rely entirely on the DM doing what you want, being passive, or by the BBEG being completely alone. If the BBEG counterspells then you have to change your plan. If the DM has minions at all with counterspell, change of plan. Honestly really just if there are minions at all it'll change the plan because you don't get to just stand there doing whatever you want without the bosses goons immediately focusing heavily on you.

Well, magic jar is fun, but requires serious prep to reduce the risks. An L15 wizard should already have a lair where they can leave their body safely, and they’d need to capture their target and bring them to their lair. This is why I chose L15 for this scenario instead of L14, because you need access to the spell. Assuming your wizard has a Researcher background or something (so you can handwave the meta), you can use locate person to find a CR12 Duergar Despot who has immunity to exhaustion. They exist in Forgotten Realms at least. Your DM might rule they don’t exist, and then you’ll have a harder time finding a humanoid immune to exhaustion to capture to magic jar into. That whole capture scenario would be an amazing multi-session mini-arc.

This is really driving my point home for me. You keep making the assumption on all of your plans that the DM will just give you want you want. That you'll have a lair, you'll be able to find a non-exhaustable creature and that the DM would allow for a quest to go get this creature and do what you want with it.

So worst case scenario, the DM says such a target doesn’t exist. Well, then you have to wait until L17 to pull this shit off. True Polymorph can create a humanoid that is immune to exhaustion (there are dozens of them!) to create a magic jar target. Or you can use Wish (also broken) to summon such a creature, or simply wish yourself immune to exhaustion.

Yes and the worst case scenario of that situation is that you fail the questline because you were waiting to gain two levels to try to build something broken. This is another major assumption that the DM and the BBEG will be completely passive and give you want you want, that being those two levels. Also if you wish to be "immune to exhaustion" there is a decent chance you're going to die considering the Wish spell is pretty clear that it is often a monkeys paw type scenario. The Wish spell doesn't say you can get immunity to exhaustion so there's no clearcut rules on it. The closest is that you can resistance to a magical effect but whether exhaustion in this situation is considered magical is a debate and left ultimately up to the DM. So you're making another assumption on whether that spell could even work in the first place.

Every single part of that plan hinders entirely on you getting what you want from the DM, DM never saying no and the BBEG being extremely passive. This would only work in theory because any DM with a spine would immediately laugh and say "Good luck" knowing that it would never happen. These types of "broken" characters are literally what the meme is talking about.