although you'll get varying opinions on the worth of watching the Spinoffs and movies, the one movie most agree is fantastic In The Beginning
(It's a Prequel). The framing story is Londo as an old man telling a B5 story to children.
Many fans advise newcomers to watch his as the Pilot and to generate their interest to see how good 5 can be. Even JMS is OK with substituting In The Beginning
for The Gathering
(Actual B5 Pilot), though, it does have some spoilers if watched as the Pilot, and I think, considering your reaction to Sheridan's Time Travel to Centauri Prime's future and Londo's flashes of future, that you are quite lucky to be watching it after you have seen the whole Series, rather then before the Series.
So, anyways, yea, you should definitely put B5: In The Beginning
on your to do list, if nothing else.