It surprises me that you guys have been playing the game for years yet you still have trouble completing a lot of the missions. The challenge in Freespace is using your head- "skills" like marksmanship, coordination, etc. don't matter very much. Every mission is a puzzle, so you should have no difficulty completing the game on Insane difficulty if you've already found the solution to each mission.
I think that the campaign's difficulty should depend on how good the player is. You can do this by branching, having an "easy" campaign path and a "hard" campaign path (if a player looses a mission, he switches to the easy path, if he wins, he goes to the hard one). You might be thinking, "well, then I'll need to make twice the number of missions", but you can just make some minor tweaks to each mission (adding or deleting a few fighter wings, for example) to adjust difficulty and have two slightly modified versions of the same mission. Also, you can set more fighter wings to arrive if the player won the last mission- you can use "is-previous-goal-true" sexps as arrival ques.
But my way of adjusting difficulty is by using secondary and bonus goals. The primary goal should be easy to accomplish- it's just there for people who want to buzz through the campaign and get the storyline. The secondary and bonus goals should be challenging. They're the ones that give you medals and promotions, while the primary goals allow you to advance in the campaign.
I don't give a solid answer to "how hard should a campaign be?" I like to make the difficulty adjustable, the way I said.