Rhino might be a good option. Unlike Illustrator it’s not a POS toy and it will import and export the vast majority of the filetypes you might throw at it. Version 6 even supports SVGs (which have been conspiciously missing).
It took me many tries to switch from SW to F360. The transition wasn’t easy, but I’m really happy that I won’t be extorted into paying their absurd “maintenance” fees* any more. 2017 seemed to be all about copying Fusion 360 anyway, so if you’re able to use SW17 you might have a leg up on switching to Fusion 360.
I’ve saved splines to DXF just fine. Some crappy software does seem to have trouble opening them though.
- $1000 a year, for anyone curious