How can I turn a iMovie video into a "proper" DVD?
Burn should still work.
There's also the old Apple iDVD &amp; though I haven't tested burning anything with it, it still appears to work on Mojave.
It appears to also still be available from Apple - https://support.apple.com/downloads/iDVD

Macworld did an article on it a few years ago &amp; came to similar conclusions - How to burn movies to disc in an iDVD-less world

They also mentioned Roxio Toast (now up to v17) as a paid alternative

BTW, I can't properly test any of this, as I have no DVD drive except in the Mac. I have to rip bought DVDs to even be able to watch them on the TV ;)