I would change three things.
First, instead of plus, which in context would mean "more often" or "to a greater extent" and usually signals that the verb is to be repeated, I would use the plural direct object plura (more things). Now the comparison makes sense.
Second, I would use a perfect tense rather than an imperfect with umquam. The perfect is much more common, since the word means "at any point."
Third, I would put the quam clause at the end rather than embedding it, since that's more natural.
Plura oblitus sum quam umquam scivisti.