I don't remember that particular mission but:
The guns on the STRKC can be destroyed, take note on which guns fire at the convoy and destroy them using lasers (the ones shaped like pyramids are easier to destroy), depending on the craft you are using disabling or forcing some of the crafts in wave to retreat can give you a bit of respite between waves.

Also the tactics emplyed depend mostly on which type of bomber you have to face, Tie Bombers are more fragile than Y-wings, B-wings and Gunboats so can either be distracted or cleared much faster.

I played when this came out on floppy disks and still do, the key when approaching larger ships or evading is to hold in the button that makes you rotate on your axis and then push the joystick to one of the 4 corners. You'll basically be corkscrewing toward the target, or away if fleeing, and they can't seem to target you. you usually have to be within 1.5 to 1.7 clicks to have your lasers hit and capital ships seem to have a range of 2 clicks so go in hit what you can, flee to 2 clicks recharge and go back in.

Yes, the X-Wing vs. TIE Fighter campaign (which was actually an expansion to X-Wing vs. TIE Fighter) is really hard. I also never got anywhere when I tried it on release. So, don't feel bad that you're having trouble. I think the campaign may have been intended for co-op online play, which might be why it's so hard (multiple human pilots are much more effective than AI wingmen), but I'm not sure.

As for advice, I'm not sure I have anything to add to what Det_Bullock said. I have heard that with waves of bombers sometimes it's best to leave one alive. If heavily damaged, it will sometimes limp back to the carrier, whereas if destroyed a whole new wave will launch. Kind of an exploit, but it may help. I also remember specifically for one of the first missions in the X-Wing vs. TIE Fighter campaign, pilots were supposed to use EMP warheads to temporarily disable a capital ship's guns so a convoy could escape. I'm not sure if that's the same mission you're playing or not. I guess the idea was to fire one off every 30 seconds, since it would shut down the capital ship's guns for 30 seconds, and then spend the rest of the time shooting down fighters and bombers.

Lastly, general advice for all the games when attacking capital ships: don't lock on with your torpedoes or rockets. Capital ships can shoot down torpedoes or rockets that are locked on. Instead, wait until you're in range (i.e. it starts the locking process), then switch targets and dumb-fire at the capital ship. They should hit, and the capital ship won't be able to "see" them. Alternatively you can try to fly really close before unloading the warheads, but that's a lot harder.

Good luck!

Haven't tried the corkscrew method yet, sounds interesting!
My own approach has been this:
- At 2km away, when attacking a capital ship, start alternating your direction continuously: put your rudder to the left, then to the right, then to the left, and so on. The frequency is ca. 1 direction change per second. Thus, the ship's lasers keep missing you. And if the ship is large enough / you are close enough, you can still pour it with lasers.

I'll have to try that, the latest edition of this game seems to have an issue with rotating on your axis while corkscrewing. When I hold that button down and move the joystick it cuts my throttle to 0. Its wierd because the default button 3 just says spin or rotate or something along those lines.

The older games had the old wing commander control scheme with button 2 that changed X axis to roll and y axis to throttle when held down, thus making the corkscrew manouver impossible with joystick only.

I think that manouver is possible only from X-wing vs Tie Fighter onwards.