clang and ECL both seem interesting, as long as LGPL license is ok. They both are complex to embed though - neither are well tuned to be embedded in other applications. (I've been exploring on MacOS). Julia could be interesting too under same questions (and a better license for commercial products) but similarly has a fair bit of glue code needed - especially if you want to restrict (say) filesystem access.

Oh right, that was a thing too. LGPL would have complicated some plans. Forgot that details.

OP: it was a two factor decision.

The first is that it seemed at the time that Scheme was going to be easier to work with, and that has proven true. It's a lot easier to do music theory hacking and integrate with Max using a lisp 1. Being able to seamlessly change whatever the first form is (function, hash-table, list, vector) with no change to the code has turned out to be really really nice in my work.

The second was that when I started I knew little about Schemes and Lisps and it seemed when going through the docs like ECL at that time was going to be a bigger lift to get going.

Solid reason, thank you. Yes, Lisp 1 for the win in many cases.

Now... there's also the wonderful Fennel possibility which distracts me when I'm bikeshedding. ;-)

Yes, Fennel is a great distraction (see my other post)!