Which mode is most important for rewards?

I agree with @Waqui here. Crystals are the most important resource. Early game they will help you level up faster and late game they will be a valuable source of buying gear/shards from shipments. Personally; I found fleet arena to be much easier to do well in than squad arena. I've been taking 1st in my shard at my payout for the last 3-4 weeks now which gets me 400 crystals a day. I get 100 from squad and 100ish from GW/dailies which gives me about 600 a day. I can spend on refreshes to speed up farms and save some for shipments. Energy for hard node farms is vital at any stage of the game as they are the longest farms and being able to get max attempts every day will accelerate those Sion/nihlus farms.

Looking at your characters; I'd say you have a good base to start building a core fleet and I'd look at building a scoundrel team that can clear the highest level of credit heist available. You can build an empire team with Tarkin, Vader, Tie fighter Pilot, Thrawn and Palp. 3 of those characters will also be immediately useful in fleet with Thrawn becoming useful there later. Boba is a great Scoundrel to start with and also has a good ship for fleet, Storm Trooper Han makes a decent early game Scoundrel tank and is needed for CLS. Phoenix have great ships for fleet and you have Biggs already too. I'd honestly look at developing those areas and concentrating on bringing in crystals and credit for now.