STIR/SHAKEN has barely started to be rolled out, and most major carriers don't fully support both protocols. Realistically the reason you've not seen a difference is that... well there's not been a change. The TRACED act stipulated that large carriers have to have it implemented by June 2021 (and several have... mostly) and that smaller carriers have a 2 year extension until 2023. The problem is that "smaller carriers" has a much looser definition than you think, which includes the phone systems used by most corporate businesses and most cheap/easy SIP providers (like NICE inContact, OnSIP, RingCentral, etc). That said, it'll be a couple years before you see the silver bullet you're looking for, and even then it'll be a couple more before all the kinks are worked out. (There's still some unanswered questions about attestation in smaller regional/local carriers. Also international partners aren't complying with the standard, so that increases the complexity.)

That seems backwards to me. Wouldn't big carriers be the ones who need more time due to the complexity and size of their systems? Shouldn't small carriers, like the ones you described, be able to roll this out almost overnight?

I want my phone to give me the option to only let authenticated calls through and give everybody else a busy signal. Today, the ham:spam ratio of my phone is worse than my email.

It's only really had any progress pretty recently. The "Traced Act" deadlines were only a few months ago, and I assume the carriers need some time to iterate on tuning it. Or maybe the FCC needs a few months of data before they can start smacking carriers around. I wouldn't give up yet.

I found installing NoMoRobo and paying the $5 a year has helped considerably.