Are crewed charter discounts possible?

Sometimes, they are. Depending on the availability and the season, the yacht managers might consider a discount. Especially with the super yachts out of the main season, the discounts are not unseen.
Please check with your broker. The way to go is making an offer. Your broker will know the yacht management and suggest the best approach (or if it is even advisable). Some managers have a no discount policy, others are more motivated to close.
One crucial thing- if you do make an offer, it’s considered binding, be prepared to close quickly. Shopping around is frowned upon and will not be supported by any industry-recognized broker.