You seem to have not really used Web Components, they are offering all what you are mentioning today:
basic infra -> lit.dev and go bundleless
"bleed-through" of CSS -> either use CSS custom properties or Shadow Parts
transport all your glorious state, like you are used to know using either attributes, properties or projected content via default slots; i.e. you have everything at your hand, that the DOM offers
placeholders -> slot fallback content
DOM nesting restrictions? / template -> Huh? Use slots and / or nested slots
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
You seem to have not really used Web Components, they are offering all what you are mentioning today: