The best attempt at a consistent canon was between 1983 and 1998. After ROTJ and before the prequels. It was pretty consistent then.
The continuity was in flux even then. Ulic Qel-Droma’s backstory as related in the Dark Empire endnotes doesn’t line up perfectly with what we saw in Tales of the Jedi, as just one example. That said, the pre-1999 EU was more accepting of artistic license than what followed afterward.