Newtongrange Folk Club has been doing one guest night a month, with the other weeks being singarounds, for many years. Seems to work pretty well. The singarounds have a small fee that helps subsidize the guests without seeming extortionate. They also do regular features (songwriting competitions, theme nights) like the ones you've been involved in. They're the busiest of the folk clubs in small towns around Edinburgh so it seems to be a winning formula. I think Dunfermline FC operates the same way, they're not as busy but just as durable.

There's a large enough concentration of folk clubs around central Scotland that transport costs aren't a big deal, they often can be arranged into a mini-tour.