[Blackthorn]

It's not free but...zw3d has full* native Linux support. You'd be forgiven for not knowing this because they only offer it on their Chinese website, even though it comes complete with a fully localized English version that you just have to switch on in the settings.

* Integrations with online parts libraries don't seem to work (don't know why they didn't bother, as it looks like it just spawned a web browser anyway), and the simulation add-ons aren't available either, but the main program itself is equivalently functional.