Shard Shop Priorities:
1. ONLY Stun Gun Salvage, but EVERY piece of stun gun salvage
2. ====WAIT until you've built up more than 1,500 Shard Shop Currency
3. Start buying as many purple g12 Med Packs as you can without ever dropping below 1k Currency
4.===> If you go over 2,000 Currency,
5. start buying any Gold Gear you need for your Arena squad without going below 1500 currency.
Always buy every bit of stun gun salvage that rotates through the shop. You'll easily get a full stun gun prototype per month if you do. In some months you can get 2 full stun gun prototypes (100 salvage), though that takes some luck on RNG because SG Salvage is rare here.