If you disregard Ibn Ishaq you might as well disregard Muhammad, since all of the biographies of Muhammad are based on the work of Ibn Ishaq. The work survived in fragments that the rest of the biographers of Muhammad used to build their own versions.
You said the magic words:
to build their own versions.
To learn the History of Muhammad one reads the Ahadith, which pretty much detail every day of his life for the last 20 years of his life.
The Biographies based on Ibn Ishaq are quite unreliable.
You may know some Muslims that find them to be Credible, but I don't know of any.
It is sad as Ibn Ishaq probably did write a very accurate Biography.