The most simple acronym would be SA. It’s unfortunately the acronym for so many different things that it wouldn’t have a difficult time establishing an identity with and for the who.
If the VOY, ENT and DIS precedent were followed, some have proposed ACA since STA for Star could fit any of the shows. (I have seen ACA in the wild on other social media.)
The quibble with SFA is that Starfleet is one word not two and it’s unprecedented to go with the first letter in a syllable to get the designation.
For what it’s worth, SA seems to be what Memory Alpha has landed on. It’s designating article stubs for the show as ‘SA Performers’ etc.
And we’re all waiting.
Not so patiently.