> can't even get Astra to consistently re-use the same font-size across all of my HTML page headings+subheadings.

This is just basic software engineering. DRY. Define the style in one place and reuse it.

This is also why you need a human in the loop, you need to make these kinds of design decisions and tell it to do stuff like this. Otherwise you're just building a pile of trash and you'll keep having these dumb easily avoidable issues.

Humans have the exact same problem. If you want a consistent solution to a problem, solve it once and reuse it. Otherwise it won't be consistent.