For JITX, we're using a language called Stanza, which provides decorated s-expressions out of the box. It's basically still just s-expressions, but it looks prettier.
LISP and languages of the same ilk are extremely powerful and esthetically pleasing for CS-minded folks, but they are also and unfortunately rather off-putting for the wast majority of technical, non-CS folks (which IUUC is your target audience).
I'd go for Python, in spite of the lack of typing: easy to integrate, massive amount of libraries, very flexible.
An at the risk of committing blasphemy, I have to mention that the EDA folks seem to have a terrible addiction to TCL, which therefore makes it something you may want to consider.
My opinion of course, the best would be to poll your target audience.
Tcl is very, very Lispy, so perhaps a Lisp is not the worst idea.
Great job giving advice, I'm sure it will be very helpful to hear from some random internet user.
Also: this is awesome! I'm a software engineer now working with hardware and built a straw man of this for myself a little while back (a sequence of python functions that generate eaglescript commands to make a PCB), but obviously it didn't even do 0.001% as much.
I checked out your site, but didn't see a way to launch the tool. How can I get access? (I'm working on a side project involving a BLE watch based around the nRF52840).
The tool is still internal for now. We're operating as a design service today, and our own engineers use the tool to produce designs quicker than other human-powered competitors.