Could he have continued to be in trade after he was knighted?

(Perhaps through a steward or somebody who took care of his businesses).

The novel says that

The distinction had perhaps been felt too strongly. It had given him a disgust to his business, and to his residence in a small market town; and, quitting them both, he had removed with his family to an house about a mile from Meryton, denominated from that period Lucas Lodge, where he could think with pleasure of his own importance, and, unshackled by business, occupy himself solely in being civil to all the world.

So it seems that it was Sir W. who thought that he should quit his business. That in turn caused that he couln't increase his fortune, and provide better for his children.