A bit of a challenge to track down, given how some pages are still using Sun/Moon information, and many of the Sword/Shield pages are still incomplete. But, I found an answer.
The culprit here is that Infestation is no longer a TM in Sword/Shield. This greatly limits the "inputs" available. The only Pokemon in the Bug egg group that learn Infestation by level-up in Sword/Shield are Dewpider/Araquanid and Frosmoth. There is likely also a pathway through other Pokemon for which Infestation is an egg move as well, so that you could chain through them. But with both Frostmoth and Dewpider already in the Bug group, that seems simpler.
As for Toxic, It's the same situation. Toxic is no longer a TM, so you have to source it in a Pokemon that learns it by level-up, and pass it around. Skorupi/Drapion is a good candidate. The only other Pokemon in the Bug egg group that learns Toxic by level-up is Vespiqueen, but that's not helpful, since Vespiqueen can only be female, and the female determines the specie of the eggs produced. Again, there is likely another pathway through egg moves to get Toxic into the Bug egg group. But with Skorupi already available, that seems simpler.
