>I've been following the Stellarium project for upwards of a decade and I'm always pleasantly surprised at how far it's come
Likewise. My first ever open source contributions were to the Stellarium project at about the age of 14. The regular volunteers (especially Matthew) were unbelievably patient and helpful. Working on Stellarium was what really pushed me to spend a lot of time learning programming.
Hey that's one of the nicest compliment I ever got from a user/contributor, thanks! (Fabien from Stellarium team)
I did some work for a company about 2 years ago to port a stripped-down version of Stellarium for internal-use on some custom ARM-based in-field devices (the company worked in the satcom industry) and I'm still amazed at the amount of complexity that goes into astronomical simulation. In fact, I recommend devs interested in this space check out the Stellarium source repo, which provides ample amounts of well-designed (Qt-ish) C++ surrounding astronomical projection, star rendering, ephemeris calculation, etc.
While this online-version doesn't seem to share too much with the regular desktop Stellarium (WebAssembly port, anyone?) it does appear to be one of the best web-based planetariums currently.