The only downside is it's stuck on supporting Python2.x so you may end up wanting to properly integrate CPython eventually. Since you're targeting running Python code that doesn't exist yet and the language differences aren't huge though, I doubt most users would mind (I wouldn't). Just an idea to consider, esp for an MVP
(One /upside/ is Jython is a Python2 interpreter fully written in Java, so the concurrency and performance may be better than CPython2 with its GIL)
Great product and very excited for this, wish I could invest in you and wish this had been around years ago when I was trying to convince Fivetran they should create reverse ETL functionality.
Nowadays, probably something like python then rust/go, just for community, and especially aligned on apache arrow. OSS async python / HTTP, with arrow dataplane support (fast,typed,standardized), is part of our bar for whether we consider a data proj as a core dep nowadays. A surprising amount of ETL startups are YOLO json for the dataplane, so we've intentionally stayed away due to reliability+perf heart pains. But maybe you can fake it till you make it that way too, and then hire staff to clean it up 2 years later :)