In my opinion, there has never been a truly satisfactory name for users of the Dark Side.
Darth was originally Vader’s first name, and giving every darksider a “Darth (Evil Adjective)” name is dumb.
Sith is slightly better, but even though it was conceived for the Original Trilogy, it has generally been associated with the prequels and their baggage concerning the “rule of two” which makes it practically worthless for general practitioners of the Dark Side. Sith has ironically become a very limiting term for a group of practitioners who reject limitations almost as a rule.
Finally, even calling someone a ‘darksider’ or a ‘Dark Side user’ uses the Jedi’s doctrinal dichotomy between light and dark sides of the Force, whereas many so-called ‘darksiders’ would probably view themselves as agnostic in their use of the Force, only recognizing it for the power it provides. This is why it bugs me that the Emperor describes his power as coming from the Dark Side, since it feels like bowing to the Jedi terminology.
If it were up to me, I’d just call non-Jedi practitioners of the Force ‘Force Users’ or ‘Force Wizards’. It should be a neutral term which doesn’t stoop to the Jedi dichotomy. There would probably also be truly committed darksiders in the galaxy, just as there are Satanists in opposition to popular religious sects, whose choice of name is often performative and intended to provoke. But these darksiders would probably be a minority, as are Satanists among the vast array of religiously unaffiliated people who nevertheless have some religious or spiritual beliefs. Some of these Force users may use the Dark Side exclusively and yet not be affiliated with the Sith doctrine, some may use the Light Side exclusively yet not be called Jedi, and others may not differentiate between the two sides at all.
Sith is the Old English word dor “Journey” which did fit with The Emperor’s, “strike me down and your journey towards the dark side will be complete.”
One of problems with Sith is in the EU it ws who the dark siders conquered, hence dark lords of the Sith. The reason Darth works is it essentially is Dark Lord in Sith and Rakatan.