I always assumed there had to be an overland route from Fornost to Annuminas. We know that Annuminas was founded around SA 3320, and that in TA 861 Amlaith moved the capital from Annuminas to Fornost. "Moved the capital", not "founded a new city", which suggests that Fornost was already a major city while Annuminas was the capital. And especially since the main north south road goes to Fornost, not Annuminas, that suggests that Fornost was a major city going back into the Second Age. I can't imagine that back during the heyday of the Northern Kingdom they schlepped down to the Brandywine Bridge, crossed the river, hiked out to Bree, and then headed back north to Fornost. So even if it's not marked on the map there was presumably and east-west road between the two. At the Annuminas end of that road there are three possibilities - either they had a bridge close to the point where the Brandywine flows out of Lake Evendim, or they had ferries (either in the river or the lake), or a road could have gone north around the lake. I'd assume the bridge option, though perhaps it collapsed at some point since the reign of Amlaith (over two millennia ago), but if so could be rebuilt during Aragorn's reign.
Anyway, if so, Fourth Age travelers from Gondor would come up the Greenway from the south, stop by Bree for a beer at the Prancing Pony, continue north to Fornost, then hang a left and go out to Lake Evendim. They'd never even have to come near the Brandywine Bridge.
On the navigation of the Brandywine north of Sarn Ford, maybe ships could sail up from the south stopping short of the ford, and then cart goods overland to a station north of the ford where barges could take them the rest of the way north.