Is spell dmg worth at high levels?

[MaxWilson]

Ok--so if you have any way to do healing that brings your damage back down below your max HP, do you get to act on that turn?

I treat it as an implicit Delay, assuming you haven't declared an action this round (i.e. were wounded on a previous round and are now back up). That is, you can still declare an action this round at any time, but you automatically lose any initiative contests to everyone else who has already declared their action and started acting. It's "implicit" because you didn't say "I'm Delaying", but in other ways it's the same as any other Delay.

Regarding the Bard: is your version of Song of Rest effectively "if you spend your short rest (or an hour of a long rest) singing, you and any friendly creatures in earshot heal 1dX damage at the end of that rest"?
If you were to use Tasha's, would the Chef bonus be the same?

No, it's "Song of Rest: you may spend HD during a short rest if a bard is singing/whatever, and if you do you gain an extra 1dX HP", the same as PHB rules.

I don't have a strong opinion on how to adapt the Chef feat to this model--it's one of the things about Tasha's that actively bothers me (mostly just because the idea of gaining temp HP from eating "treats" just seems stupid) so my personal answer is "don't adapt it, just leave it out of the game." I do let Healer work normally though--my whole agenda is about preserving the narrative weight of wounding in the default case, but I don't mind a specialized healer getting to be freakishly good at healing wounds. That just makes everyone go "wow!", whereas making deadly wounds automatically heal themselves over the course of 1-4 hours just makes it seem like an alien planet where no one can even relate to the concept of long-term wounds.