Seconding GMAT. It's a distant third in the space in terms of adoption (behind FreeFlyer which is itself (anecdotally) far behind STK) but as a GN&C contractor I've used GMAT over FF and STK for a few jobs.

Particularly when it comes to code generation it has, IMO, a big advantage over FF and especially over STK. It's relatively lightweight, at least in my experience it's much faster to spawn and destroy instances than FreeFlyer, and it's pretty simple to write code to leverage GMAT to do interesting things.

Two things I've used GMAT for that would have taken longer in FF or STK are generating thousands of randomized ephemerides with keplerian parameters distributed in a set range, and automating a Monte Carlo-esque analysis focusing on error magnitude permutations by generating GMAT code from templates and collating into a single script file.

Not to mention it's free; STK and FF are very much not.

I'm on a first name basis with a chunk of the GMAT development team, though it's been a minute and I don't know how they're faring given recent political changes - they were out of Goddard which was having issues even before this January.

I don't know how often they accept code from the public, but I know they appreciate bug reports.

Anyways I guess I just wanted to go on about GMAT a little. It's a really cool tool and it's awesome that it's free. I wish it had more industry adoption.