You could start the movie halfway through episode 3 when Ahsoka and Sabine drop out of hyperspace near the planet where Morgan is at with the map. Here is a crawl that could work, also better explaining the state of things so Rebels won’t be as much of a requirement:
It has been years since the heroic Jedi Padawan, Ezra Bridger, and the menacing IMPERIAL GRAND ADMIRAL THRAWN went missing during the Battle of Lothal. During their exile, the EVIL GALACTIC EMPIRE has fallen, but rumors are now spreading of Thrawn’s return, which would galvanize the IMPERIAL REMNANTS and reignite war throughout the galaxy.
Sinister agents have infiltrated the new fragile government and are already at work to undermine the newfound peace. Failing to convince the NEW REPUBLIC to act, AHSOKA TANO, former Jedi Apprentice of Anakin Skywalker, has taken it upon herself to investigate this emerging threat.
Forming an uneasy alliance, Ahoska has recruited a former student and friend of Ezra, Sabine Wren, to assist in her quest. They have tracked the location of a recently escaped prisoner, MORGAN ELSBETH, who is in possession of a secret map which is vital to the enemy’s plan…
It is way too long for a Star Wars crawl. Usually they are shorter and more concise.