when is the best time to worm pigs after weaning, and what is the best wormer to use?

We worm ours at about 5 weeks and use ivermectin.

I'll also worm mom about 2 weeks before farrowing.

You have a few choices when deworming young pigs. Ivermectin is one option but safeguard in the feed is a much broader spectrum dewormer. If you are satisfied that large roundworms are the only issue you can use Banminth and feed it continuously for several weeks at a very low cost and have the ability to combine it with mecadox.