wow that's so disgusting...
First, I can understand you that you must be completley upset as I have been in a similar situation as you, except that I lived in a HMO and there was another stranger who was not upto hygiene standards and I had to clean this subjects piss every day from the toilet seat...
My approach was to confront this subject and make him aware that we both use the downstairs shower / toilet and that there are regularly piss marks on the toilet seat. Since then the situation worsened and there were many other antisocial behaviour displays like playing loud rap music in the middle of the night with full bass on so that my walls next door are trembling, or on another occassion tempering with my private food belongings when I found dirt inside my milk, insulting comments ...
I have also reported him to the landlord, or it's representatives the landlord's agency who reacted quickly by sending a round email to all tennants reminding everybody of the house rules.
I havn't been treated like that before in the past and it all started when I moved into this house.
I can only imagine that this gentlemens social and family networks must have thought him to treat others in such a manner ... on the other hand, another occupier in this very same house verbally abused and accused me of colonialism and slavery the very next day after I moved in ... I don't want to tell this story here now, but it had a very bad impact on me as well ...
My position and case might be similar to your's but at least I have the opportunity to move out since I am only a tennant at this property.
I have made a horrible experience and I am happy when I have left this place and don't need to see or have contact with this subject any more.
To come back to your story and the revenge part with the toothbrush, it should never escalate that far because at this point it also impacts your mental health in a bad way. Basically you are lowering your moral standards to his low standards which you tried to avoid in the first place.
My piece of information is of general advice like obvious things, approaching him to make him aware that you feel bad if the toilet seat is not clean. If that doesnt help then to have a discussion with the wider family to address the issue about keeping bathroom areas clean for the whole family to enjoy.
All the best, J