Monster Zero is a prequel to Destroy all Monsters. It kind of makes sense in terms of progression for humanity to master space flight and have advanced technology after their interaction with Xiliens.
The only reason continuity is messed up after this is due to Sea Monster. It's not a super big deal, but I'm always curious how Godzilla ended up in that damn mountain.
Son of Godzilla clearly is in the timeline because of Minya and DAM.
Continuity gets really messed up Hedorah onwards, which is why I'm advocating for it to be spun off.