I think the intention of a stronghold is to create something that PCs can spend their cash on, not to generate cash in itself but there is a point that it does become self-sustaining but I don't think the rules really cover either generating income or taxing the local populous.
I would say a stronghold in itslef can't charge taxes, that is more to do with a recognised nobleman that owns the land that peasants live on.
Then how does a party manage it? I mean, the treasures are so small in FL, I can't even begin to imagine how a party could ever afford the barest minimum. If they don't have some sort of income stream, leaving a guard for a couple of weeks is prohibatively expensive, not to mention the other hirelings.
Using the Raven's Purge as an example of the sites, players simply cannot gain the resources to ever hold or develop a stronghold without making money outside of adventuring.
Also, while I would generally agree with you vis-a-vis noblemen and taxes et al...but the post apocalypse nature of the Raven Lands basically does away with that conceit. What you have is gangs and petty warlords. There are no 'kings' or 'nobles' left, and those that could claim that title rule very little directly (Zytera, et al). Maybe the dwarves and elves have working political structures but humans really don't. Consequently a bunch of peasants in a village probably would have to pay protection money (which is really what early taxes basically boiled down to). That being said, I see no reason that a party of combat heavy reavers...er, adventureres couldn't take over a village if they wanted to and force the peasants to serve them...basically as every political entity did throughout ancient history.