Ok, so first of all, NEVER buy character shards for crystals out of the shipments. its a big waste of your crystals.
if you want a character, you will have to farm him from the respective node, and yes, that can take a long time.
Use your crystals on energy refreshes (the first 3 refreshes for normal, ship and mod energy only cost 50 crystals each, and not only will that accelerate your leveling by alot, it will also help you get more characters/gear. Cantina energy refreshes cost 100 for the first 3, but they are also worth it, becouse you get cantina shop currency as well, and you can buy characters from that shop)
now that we have that out of the way, you want to play a sith team, but for that to work, and help with your sustain issues, you should get emperor palpatine, he is one of the best Sith leads, and fairly easy to get.
To get him, you are gonna need 5 rebels. I would recommend getting the phoenix team for that (Hera, Ezra, Zeb, Kanan and Chopper), thoose will unlock you EP and then later also unlock you thrawn. and they are also great in galactic war.
So for your other characters you right now try to use: sidious sadly is a very very bad character, and he wont help you much in the longer game, so putting many ressources into him will hurt you, when you notice you dont have the ressources for a different and beter character. same kinda goes for savage.
Dooku has some potential and isnt a bad character at all.
So once you get EP, the Team you are gonna use for a while is gonna look like this:
EP, Vader, Dooku, Tarkin and Thrawn (the lead from EP also works for empire characters), and this is gonna be a reasonable strong team for a while.
Also you are gonna unlock fleet soon, so you should prepare for that, you will need 5 dark side ships at 4* to unlock tarkins capital ship at 5*, and you need that ship, to unlock the zeta challenge, and for that you need 5 ships at 5* (they also can be light side).
So id look at the following ships and pilots for that:
Geonosian Soldier + Ship
Geonosian Spy + Ship
Sun Fac + Ship
KRU + Ship (both on the same node)
Vulture Droid
If you wont get vulture to 5* by lvl 78 (thats a possibility) you should probably just farm buy ahsoka tano shards from the cantina store, and get her ship to 5* instead, alternativly you can just farm the regular tie