How can a wizard in d&d 5e produce effects that cannot be avoided?

I have been playing a wizard in a fairly long running campaign, at least by my standards. There have been a lot of good moments but after my third attempt to make disintegrate work, I have realised something: everything worth using a big spell on probably has a good enough save that the big spell won’t work.

Obviously, I don’t like that, so I have come to ask, what ways can a wizard use spells that are guaranteed to have at least some effect?

The suggestions are ideally achievable by 15th level, since that is the goal of the campaign,and should not require more than a dip into another class, hopefully keeping pure wizard.