Pages just crashes if the IBA file is bigger than 170 pages or 40Mb. Clearly the testing regime at Apple Pages developer community needs to go back to the drawing board. I tested it by importing a chapter at a time from Word or Pages files and tested it periodically to see if Pages could open it, sixteen chapters and it bombed. This was on my MacBook Pro as well as on my M1 Mac mini. So the issue is with the latest version of Pages.
As for editing a book in Pages using an IBA template, forget it. It is so limited in relevant features as to be a joke. e.g. No horizontal scrolling within chapters. This is a pure token gesture by Apple, not fully tested and lacking key features.
My brand new M1 Mac mini is only 3 days old and currently parked in the corner and disconnected. It simply doesn't do the job. So I'm back to my 6 year old MacBook Pro and living in hope that when Rosetta 2 comes along I can at least run iBooks author on it in emulation.
My fear is the bad old days of 9 years ago when iWeb was withdrawn will return and we will be losing iBooks Author as well.