My personal dumb and useless head canon for what happened between “twilight of the apprentice” and the epilogue for Ahsoka goes thusly:
Ahsoka realizes that, now that everyone left the planet, she is stranded on a deserted planet with no visitors and no ships. thankfully, she is a great mechanic, and manages to scrounge up a ship from old parts strewn about from old, destroyed ships and the temple weapon. some are thousands of years old. some are much more recent, from dumbasses who crash on planets... like her lineage.
she explores and studies the temple and the sith writings in her spare time (entertainment is scarce) as she constructs what is, without a doubt, the jankiest, most engineering-student ship that has ever flown. her belt, an old ration wrapper, and tape, among others, are integral parts of the engine. it flies on luck and the force. Morai is facepalming. Bendu is kind of impressed
she nonetheless uses it to get out because she left her self preservation somewhere on christophsis and they do not have a lost and found department.
She then goes to the closest planet to find replacement parts, upgrades, as well as luxuries for her ship (she was holding her pants up the whole takeoff and then decided to just throw them off once inside) (and she needed a kitchen, and food) (and this ship is so old in it’s apparent design and mismatched she easily fits it up to be invisible to the empire).
Knowing that she cannot return to the ghost, or Rex, nor could she ask for another confrontation with Vader, she decides to be the biggest pain in the ass for Sidious, because why not, and also she hates the bitch.
She destroys force-sensitive ‘sith nurseries’, corrupts secret research data on under the table experiments, just attacks Sidious’ Sith agenda that the rebels don’t care for (cuz the jedi are dead but they forget that the force is not).
now Sidious knows who it is, but cannot send Vader again, because he is busy, and also failed once and he does not want Vader to be exposed to Anakin again, and Anakin never hated Ahsoka unlike Obi-Wan, so he has to go himself. of course, he is an asthmatic Grandpa, and she is young and fit, so she was gone before his ship arrived on site, but she left cute messages!
she, head of rebel intelligence, skilled commander and general, war veteran, and he, wartime emperor, are basically just raiding places and chasing each other but because of their own skill and experience, the strategy needed become more and more complex until they are like playing 5d chess whilst the rebel alliance is playing checkers.
(still mad that they gave military titles and rank to Senators and politicians with no military experience or training. do you think military commanders would give half a second’s thought as to weather or not they should retrieve the death star plans? or even refuse to because they don’t have the votes? nah, they would see planet killer and see it as civil war ender and send everyone they can to destroy the thing the second they learn about it)
and that is why sidious does not actively do anything more than sit on his throne and walk to the window to stare ominously in space next to vader in the OT. he sends vader off then goes to try and fight and mange anakin’s wayward apprentice.
and then when she goes to the cloning facility that is preparing for palpatine’s ‘return’, she programs all the droids to implant biochips that would slowly cut his connection to the force and kill him the more he uses the force, because that is karma for what you did to the clones and order 66.
and that is also why she does not go fight him and lets Rey do that, because she is completely alive, what are you talking about, the daughter made her immortal unless killed and my girl is too strong to be killed. She’s just off being badass in another war or force plane, and is not interested in coming back for a little first order uprising, that’s it.