I would use zoom levels: pixel speed is kept constant for moving the reticle, but powerful zoom levels (2 or 3 levels) allow to have a different angular speed for reticle.
Want to precisely aim at that slippery target? Zoom in. AFAICT most people zoom in during combat anyway (and it gives a better view of those lovely ship models).
Want to point the other way? Zoom out.
There could also be a mouse sensitivity high/low toggle, but with zoom levels, I'm not convinced that would be necessary.
Btw, the zoom isn't strong enough in the prototype as of now IMHO.
Interesting idea, I'd have to try it out to judge it, though.
Maybe acceleration could be at the edge of the screen for agile interceptors, with a large surface of precision aiming. For sluggish corvettes, there could be a smaller circle of direct aiming, with larger borders for acceleration, possibly higher at the edges of the screen than right next to the circle.
Another idea for the sluggish Corvette:
Maybe the corvette should be able to toggle between a (3rd person) Capship-mode and a (cockpit view) Fighter mode.
Here is a possible Capship control scheme:
3rd person, centred on the Capship, move the camera with the mouse. The mouse pointer also serves at pointing at things for giving orders.
Like in Kerbal Space Program, either with the same "move camera when pressing LMB", "always move camera" or even the inverse "cursor mode when pressing LMB"
Like small crafts as mentioned above, it has a directional pointer, toward which it is trying to point. This reticule may include roll level, and discreet 360° compass lines, so you can always see your planned (and maybe current) direction(s) even when looking down or sideways.
When pressing a key, the mouse moves the reticule along with the camera (it doesn't automatically centre on it, if what your side or rear is facing is more important than your direction by itself). Maybe you see an angular speed and ETA (useful feedback for beginners).
Speed would be set with the WASDRF like with fighters, but they would be used as throttle: press longer to set a higher speed in this direction. Maybe the longer you press, the higher you accelerate. Maybe there would be a "cancel acceleration in(against?) this direction" key, combined with the direction keys.
Maybe there would be a "direct thrust" key, when you just want to run the engines in a direction, and/or a "flight assist off" equivalent.
This could be useful to give orders to the Corvette turret, against interceptors it has little hope to lead with its main cannons. The Fighter mode would still be useful against slower crafts like capships, maybe other corvettes or even bombers.
Maybe the speed control should be kept in Fighter mode, though, if the Corvette accelerates well enough that this capship speed control wouldn't work.
This would allow to keep the Corvette big enough, helping bridge the gap between small crafts and capships. A smoother size progression would probably help making gameplay more varied.