imbecile

quick googling and the AI summary I fucking hate but seems reliable enough here

a person who acts with extreme foolishness, stupidity, or a severe lack of good judgment

barring the inclusion of the afforementioned "stupid" this seems fine.

While used today as a casual insult, the term has historical roots as a formal clinical classification.

I'll have to do further investigation but it might have the same baggage as those other terms.

I don't know why it is so hard for some people (not you) to shed reactionary terminology. Either because they are too lazy to come up with something more clever or are defensive because they regularly use such terms.