That's a hard call, Dr Strange being one of the most "volatile" in terms of power in the Marvel Universe.
First and foremost: the largest part of his power does not come from himself, but from the huge amount of artefacts he owns (and spells he can cast, too). Which is always easy to mess up for comics writers, as those artefacts are mostly "channels" for some magic entity power, so as soon as Dr Strange gets a cold or spends too much time at the disco, he could be too tired to properly ask the entity for the power.
Just to put things in perspective, Strange can easily destroy an entire reality because he doesn't like it, or fight the most powerful Abstracts (like the In-Betweener), but he cannot cure a cancer or even cure a bullet wound. And, he's considered (by the aforementioned Abstracts) one of the most powerful beings in the main Marvel reality, but he gets easily beaten by Hulk and admits being less powerful than Sentry; no need to say that Hulk and Sentry could easily be obliterated by any Abstract or even most of the gods.
But, coming to your question:
at its peak [...] exceeded
The key is all in the bold part: in the past (long distant past, Dr. Strange #4, 1974) Dr. Strange was able to merge with and become the avatar of Eternity, our resident manifestation of time and reality. Eternity can easily kick the butt of our big, purple,-horned planet-cruncher and, needless to say, can easily stomp the Celestials too (after all, Eternity created them with just a thought).
So, how powerful is Dr Strange? Nearly not at all, but he's very good at bargains for someone else powers :-D
Does it count as "having more power than Galactus and the Celestials? Well, that's debatable, but in a sense it's true.