The quickest way to have high stars for five heroes is to farm them in different locations so that they don't compete with each other.
I would suggest the following heroes from the different areas of the game:
Luminara in GW
Qui-Gon Jinn in Cantina shipments
Mace in Cantina battles
Eeth Koth in Arena
Jedi Consular in LS/DS hard battles
Luminara unlocks at 3* with 50 shards. To promote to 4* u need 30 shards. From 4* to 5* 65 shards is needed. From 5* to 6* 85 shards is needed. From 6* to 7* 100 shards is needed. Total is 330 shards from none to 7*. If u can complete GW each day u get 1.2k GW tokens which means 15 shards daily and u can get a 7* Lumi in 22 days. If u can get 800 GW tokens it's still ok, as 10 shards daily means a 7* in 33 days.
Eeth Koth unlocks at 3* with 50 shards. To get to 7* u will need 330 shards. That translate into 330/5= 66 shipments which equal to 26400 arena tokens. If a rank of 21-50 is maintained, in 35 days there will be 26250 tokens. A few days below rank 21 would be needed in order to get 7* within the event time.
To get QGJ to 7* from scratch u would also need 330 shards which means 66 Cantina shipments, which is equal to 26400 Cantina credits. Each battle gives 17 credits, so u will need 1553 battles, at 10 energy each that would be 15530 energy. Each refresh is 120 energy, so 130 refreshes is needed. 130/35 = 3.7 refreshes per day. Taking into consideration natural cantina energy build up and leveling rewards one will need to buy the 100, 200, and 400 crystals Cantina energy refresh to be able to get QGJ at 7* in time. Those who started farming him since the previous chromium only toon became farmable have a headstart.
With 1553 battles, if there was a shard rate of 25%, 388 shards would be obtained which would be sufficient to 7* Mace. Mace's shards can also be farmed from hard LS battles.
JC should already be 3* by the end of the lower levels. It would be a matter of farming the hard missions and using Cantina to supplement the remaining shards.
Good luck to all and happy farming!
May RNGesus be with u.