Who cares that it took a dozen years for loot drops to match Red Vardo gear? They do now. If it makes sense for any factions to invent cross-country banking, it's the trading factions -- just like in real life. "Further cost-benefits?" That sounds exactly like an IC reason to do it! Taxes aren't a problem, they just factor into the final %, which is really, really low in Barovia to anyone with low OCR.
When you start objecting to ideas that make complete IC sense and then suggest that Degannwy, an elven grove, should have a bank, you need to take a step back and realise that you're approaching this from far too OOC a perspective. (A bank in Dvergeheim probably does make sense, though.)