Dubbing American actors except Don Frye

They don't always overdub the American actors. The English version of Monster Zero retains Nick Adams' English dialogue. But the reason is usually that Toho doesn't actually try and find actual actors for the parts of American characters in their films, they just find random people in Japan that look American and speak English, even if said people are British or Australian. Usually because these people are not actually actors, their acting is pretty bad and their dialogue sounds stilted and awful. Japanese audiences will hardly notice this because these actors are speaking a different language, but for the English versions whatever company handles the dubbing will usually overdub these actors so it sounds less atrocious. The reason they didn't overdub Nick Adams was because he was a respected Hollywood actor, while Don Frye was a popular MMA fighter who despite not being an actor had more competence in his single acting performance than the terrible English-speaking actors in other Godzilla films.

^Truth

Also Toho had better relations with American studios during the Showa era. Thus they ended up with better/actual actors delivered from those studios even for minor roles. Monster Zero, Frankenstein Conquers the World, War of the Gargantuas, Dogora, The Mysterians and Battle in Outer Space all featured actors that were found with the help of American studios/distributors due to international interest in Toho science fiction.

The international market was less interested in the Heisei and Millennium Godzilla series and despite Toho's efforts to push them into the North American market, no one budged after 1992 until the late 1990s.

^ Which is a shame because in the Heisei series Toho really tried to push for a more international element in their films and featured considerably more English-speaking characters than in the Showa era. Ultimately this just made the acting seem really really poor.

It was a push to appease the international market the collapsed on itself, because Toho didn't have the resources to find English speaking actors with a grain of talent for these films.

The age of "realistic" special effects had been kicked off with Star Wars. The Golden Age of kaiju films were over. International markets weren't interested in them anymore. It's also been speculated that the tight continuity of the Heisei series made it difficult for distributors to market outside of Japan. Up until then most of the Godzilla movies were fairly loose and stood on their own without any real connection to a previous film. The Star Trek-like continuity of the Heisei series apparently scared some distributors away who didn't want to commit to buying more than one or two films.

This may explain why there was a 4-7 year gap between the 90s films' and their North American releases. It would also explain the lack of continuity in the Millennium series and why they were picked up so much quicker.

^ Yep. That pretty much sums it up.