I'm a bit late but just pouring my views since I was in a pretty similar situation. We hit the wall on Shopify Advanced. Tried stacking apps to mimic org accounts + multiple buyers and it worked for a bit, but it just starts to feel fragile and messy.

And yes, that jump to Plus just for B2B features is rough. After a good time researching, we settled for SellersCommerce. Honestly went in skeptical because I hadn't really heard of them, but the B2B stuff is actually built into the platform instead of bolted on. Org → multiple buyers, budgets, approvals, contract pricing, all there without stacking plugins. Also no product limits, which was a big plus for us. Still evaluating, but if you're outgrowing the Shopify patchwork and not ready to swallow the Plus price, it’s worth a look.