Are there any serious LaTeX competitors in terms of comparable vocabulary/document layout power, but better programming experience? Trying to understand e.g. variadic macros and my head is spinning.

Perhaps HTML + MathML could have grown into a contender if the latter hadn't been defenestrated.

You could try LuaLaTeX, which is LaTeX extended with the Lua scripting language.

Lout (https://savannah.nongnu.org/projects/lout/) looks promising but also a bit abandoned.