Writing Software That Will Still Build and Run in 20 Years

In case the author is here: did you consider Tcl? Or QuickJS?

I’ve written maybe 20k lines of Lua (and embedded it twice), but… I just don’t want to deal with its idiosyncrasies (especially one-based indexing and loops) anymore.

If I had to embed a language today, I’d probably try QuickJS first. Tcl also sounds similar to what you described (although I’ll admit I’ve only written trivial amounts of Tcl).

Edit: I ask because I’m also interested in a simple runtime I can maintain myself (if needed).

Write it in Common Lisp. I maintain open source software written in the 1980s. It still runs unmodified.

Unlike languages with "standards" that change once a year (I'm looking at you, C++) the Common Lisp standard is still "standard".

No mention that 64bit ntp time will roll over in the year 2036, 32bit unix time will overflow in 2038 and the next GPS week rollover is in 2038.

The year 2038 is fifteen years from now.