Each new Information Book adding stuff to the Kiryu saga timeline is just making it bloated now and implausible
Seriously, Showa Ghidorah and Gigan attacked earth in this timeline? Mothra had been in Japan only a few years earlier and the city wasn’t plastered with merch?
The Godzilla X Mechagodzilla Completion (ISBN: 9784798613536) lists a history of the self defense force as it relates to monsters for Godzilla Against Mechagodzilla (2002) and Godzilla: Tokyo S.O.S. (2003). This includes citing the battle against Kumonga and Gigan by the Type 66 Maser Beam Tanks, while also noting the SDF's confrontation with King Ghidorah in 1999 with the following passage: "1999: Together with Mothra, with whom the SDF had once fought against, King Ghidorah was annihilated."
Godzilla X Mothra X Mechagodzilla: Tokyo SOS Fantastic Collection, I think did give an incomplete picture stopping before the 90s . And some more recent attacks on the history would help justify Kiryu’s construction, adding explicitly Alien kaiju in seems...not fitting for the established tone. The Space Amoeba works well because there’s only some witness testimony, and probably goes down as hard to prove.
1954 – Godzilla appears 1956 – Rodan and the Meganulon appear 1958 – Varan appears 1961 – Mothra appears 1962 – Maguma appears 1963 – Manda appears 1964 – The Dogora appear 1965 – Baragon, Frankenstein and the Giant Octopus appear 1966 – Sanda and Gaira appear 1967 – King Kong, Gorosaurus and the Giant Sea Snake appear 1970 – Gezora, Ganimes and Kamoebas appear 1987 – Kamoebas appears
And Ghidorah having a showdown in 1999, the same year the second version of Godzilla shows up?