Ditch that email address.
Create a new domain (preferred option) or use one provided by an alias provider (like SimpleLogin). Move all your sites to their own alias. At this point you should be getting no spam at all. If you do though you'll know which site it came from as they have their own alias. You can simply disable that alias and no more spam.
You don't need a spam filter anymore!
Not any plan. I'm on Mail Plus and that doesn't include SimpleLogin.
I signed up for SimpleLogin prior to ProtonMail. I don't need the extra stuff in the Unlimited package.