My delicious, chocolatey munchkin core always wants to red-line the system, see how far it can go - and then the GM candy coating around that core tries to fix it.
*snip*
Not the most munchkin thing, though; apparently a Druid (Moon Circle) has effectively unlimited temporary HP at 20th level. They can transform into a CR6 beast as a bonus action, can do so unlimited times, and each beast form's HP acts just like temporary HP. Mammoth is the only CR6 beast, but hey, who doesn't want to be a mammoth? 130 bonus HP a turn, and a gore attack or spellcasting?
I think you're munchkin-pushing this a little bit too far. Yes, you can transform as a bonus action, but that doesn't mean you get to constantly "re-wild-shape" into the same thing over and over again, at the beginning of each turn, to "renew the temp HP pool" as you're calling it. Also, you only get one bonus action per round. Therefore, you can't shift, cast a spell, and then shift back in the same turn. The only thing you can use spell slots for while in "wild shape" is to regenerate hit points.
I think the situation is an oversight, on their part, however . . . to at least some degree. On the other hand, as a GM, there's one easy counter to that, if someone went to abuse it . . .
Bestow Curse. It's such a simple spell, but it is flexible enough that I'd certainly use it to curse someone to be stuck in their current form until they're beaten out of it . . . or to prevent them from shifting in the first place. Even if the character has remove curse, I get to mess them up some before they remove it and take on a wild shape, gaining their "temp HP pool". You can also turn around and pummel the druid with debuffs that will persist even between shapes (temp HP mean nothing if I hit you with so many rays of enfeeblement that you die of being reduced to 0 STR), or hit them with an Anti-magic field to prevent shifts, etc.
Of course, by that point, you're level 20, so you should be pretty beastly, anyway, and it should be really tough to kill you. Regardless, I'm not saying all the stuff as, "I'm a jerk GM who enjoys breaking and killing my players' characters", I'm saying all the stuff as, "If someone is going to munchkin my game too hard, to try and break it, I'm going to GM-pummel them like the fist of an angry god." I love GM'ing, and I love it when everyone has a good time. When one person breaks a game, it tends to ruin the game for everyone except the person breaking it and basking in everyone else's misery/aggravation.
The only players' characters I tend to kill off happen either because . . .
A: They're pushing the munchkin limits hard enough that they're trying to break the game (I don't mind some munchkin play . . . everyone does it at least a little, otherwise we'd have no desire to raid the crypts and dungeons of long dead people and ancient civilizations for the purpose of getting a better magic sword).
OR
B: Stupid hurts. If players do something that is just utterly senseless, reckless, and asking for trouble, then they could very well die.