Technically you still have a web of trust with S/MIME. You just don’t say “I trust you because X said you’re good and I trust X” but you say “I trust you, because you paid X money and X did probably a good background check on you”. So rather a tree than a web.
I guess it is philosophical to argue if a tree can be considered a net as well.
Not exactly. Maybe you benefit from an additional virus scan (if Proton does this). What you certainly benefit from is the “only load external resources when told to” feature. This prevents tracking since loading the external resource == the mail was opened.
What exactly do you want to achieve in terms of security?
I think they come with ublock itself and are called “annoyance filters” or something like that.