Your first goal should be to get Tarkin's ship and 5 other Dark Side ships to 4-star. You will need these to upgrade Tarkin's ship to 5-star.
Your next goal is to get your 5-star Tarkin along with 8 other ships (can be light or dark ships) to 5-star for the ship ability material challenge. This is the challenge that drops Zeta mats.
After that you can worry more about a cohesive Fleet Arena strategy.
By the way, it makes sense to have those 4-star dark side ships pull double duty as some of the 5-star ships needed for the Zeta challenge.
Also note that there are not a ton of dark side ships, and even less that are easily farmable. I would suggest Slave 1 (Boba), TIE, First Order TIE, Sun Fac, and Geonosian Soldier ad your dark squad. Based on the characters you have Biggs and Jedi Consular have easily farmable ships to make your 6th and 7th.
Consider Ahsoka for your 8th: She fits into any fleet and you'll need a team of Jedi built up to at least 5-star with mediocre-or-better gear eventually. She is useful for Jedi needs, for almost any fleet, and she is farmed from Cantina Shipments.
So if you want to buy a starter pack you could buy Boba/Slave 1 (he is a good character anyhow), but you could also farm Slave 1 from the Fleet Shipments (It is a rare) and farm Boba from Cantina Shipments.
If it were me I would spend the 40 bucks (or whatever it is) elsewhere.