Bitta follow-up here for completionist's sake.
I snooped my save file with Mouseypounds' Stardew Predictor to confirm how Sandy's Shop works and yes it seems that
almost everything is available from The Oasis if you choose not to go the Treasure Totem route (especially if you don't have enough resources to craft them!). The only things missing from Sandy's Shop are the 3 Rare drops mentioned earlier:
Tie Dye Shirt
Neat Bow Shirt
Red Buttoned Vest
Everything else will
eventually show up if you're diligent and patient enough to check every day (after getting the Desert Obelisk as the Bus isn't worth the wait and daily 500g). She's even open for the Feast of the Winter Star!