Frame challenge: Don’t clean the board yourself
Rather, it should be your students whothat clean up when they don’t need your notes anymore. This can be right at the end of the lecture, or while they discuss with you afterwards, or after you left, or before you come in, or even as you are about to start your lecture.
Whatever suits them and lets you use the board is fine.
Now of course this depends on your students cooperating. Hopefully you have a good relationship with them and it just works. A simple means to get them motivated is just not to start until the board is clean and cut into Q&A time to compensate.
Or do it as one (in-)famous famous lecturer here does: Every new batch of students would miss cleaning the board once – and only once – - and only once as he would just write on the dirty board anyway. After all, the writing was for the students, not the lecturer.