Frankly, I think William was a stronger person than anyone, himself included, gave him credit for. Sure, he was a bit of a ponce, and a bloody awful poet, but he loved deeply and unconditionally, and that affection burned into him, even when he became a vampire. I think, free from doubts and insecurities, -- and, of course, conscience -- William's personality was remarkably strong, enough to make him largely anomalous among vampires, and strong enough that, when he gets his soul back, he doesn't suffer the drastic personality shift Angel does.
I can agree that William had pronounced character traits (feeling passionately about things), but I don't see the sort of determination and agency that strikes me as characterizing a "strong" person. He appeared to be almost entirely under his mother's thumb, and in possession of a desperate need of approval from Cecily and his peers that transferred to Drusilla once he met her. After an initial transition period his vampiric personality didn't seem to bear much resemblance to his previous human one aside from having a romantic side.
Which makes me think, the respective sires may have had more to do with the development of Spike and Angel's vampiric personas than I'd previously considered. Darla was attracted to Liam for his devil-may-care attitude and joie de vivre, and those traits played a large role in his postmortem behavior with an added heaping of evil. Drusilla seemed drawn to William by his romantic nature, which—at least as expressed toward her—continued to be part of his makeup even though it was very unusual among vampires.