Surely they had to pay her a full guest-star salary as it was the fifth episode she was in.
I'm not so sure about the pay scale. The standard guest star is brought in on a single episode basis. Some of them, like James Sloyan, are brought in to play the featured guest character, in episodes like The Defector, or Firstborn. Others, like Andreas Katsulas as Tomalak or Natalia Nogulich as Nechayev, are recurring, but still seem to be optioned one appearance at a time. Even Sito Jaxa comes back for Lower Decks, but it's not like she's been regularly recurring.
Then, there seems to be the ever fleshed out supporting cast. Some are nonspeaking extras like Lt, Jae, or barely speaking like Martinez (in sickbay) or rotating helm officers like Ensign Gates, who are in a ton of episodes, superficially.
Some of these get optioned for more, & are developed, like O'Brien & Keiko. Ogawa was eventually added to this roster, later on too. I'm not sure, but I think they're contracted differently, & I think Michelle Forbes was in that grouping, (& maybe Dwight Schultz but I'm not sure) Only they brought them in specifically to do so, which is where they get the impression this character will grow & become more central, in time
I suspect there were others that they'd intended to do similarly, like Ashley Judd's Robin Lefler (who is supplementally around in Darmok, before she gets featured in The Game. Also, maybe Sonya Gomez, who is introduced rather flamboyantly in Q-Who, & is still around in Samaritan Snare. For whatever reasons, those didn't pan out, but Ro landed especially well in the narrative, & was ripe for more, but just like with Ashley Judd, they just couldn't nail the actress down
Forbes had first been an actual guest actor as David Ogden Stiers' daughter, in Half a Life. She's so damn good in it, it must've caught their attention, & they revisited her for a more regular contract, the next year