More Words, Deeper Hole

But he didn't know about the Second Foundation until he started talking to the Foundies, did he? Once he found out about their existence, yes, they became Priority Enemy Number One, but his plans for making the entire galaxy bend its collective knee to him pre-dated that.

I think it's more that his motivation was essentially revenge -- the galaxy had been mean to him, so he was going to knock it down and kick it in the face. It didn't seem like he had thought much about what would happen after that.

(Granted, most people could be excused for thinking that when the first entry on your To-Do list is "Conquer the entire galaxy," then you probably don't need to worry too much about what the second entry is.)

It seems entirely possible that, if there had been no Second Foundation, he would have steamrolled the galaxy into his own personal empire, and then almost immediately gotten bored with it and moved on looking for something more challenging.