The paid proton accounts let you use several custom domains, although I'm not sure if you can combine custom domains with email aliases. For random sites the email alias with the stand @proton.me would probably suit your needs.
After about 3 years of use I've been very happy with proton's spam filtering.
I use proton coupled with my own domain and SimpleLogin (which is now merged with Proton) to create infinite custom email addresses in my domain. I then use filters to move the recipient address to folders and I can turn off one of my SL addresses if it's compromised or sold and create a new one.