I play the DA campaign now on "normal" with a similar strategy with quite good success, so I can't tell you much about the DL campaign, but I can give some advices...
You have to get yourself in a position where you will not be forced on a defensive. Your soldiering should be pretty darn high at some point that AI can not just rush your planet and expect to conquer it with one transport. By the time they try to send a second transport, you should be able to send defenses. I never have more than one ship on a planet, and that's usually a defender. If that planet even falls, I should be able to take it back easily. Dread lotrds seem to leave my planet in a pretty nice state though if I take it back
If you have to deal with dreadlords themselves, you can usually build 4 very fast cargo hull ships with 1 damage and good sensor range to surround their planet and watch for transports coming your way. As soon as you see a transport, intercept it. Those tend to survive the encounter because the game considers them "stronger" even though Dreadlord transport has 7 damage (also 1 hull).
If you have a strong opponent, even the dreadlords you can place a military starbase at a strategic location and own their incoming ships because they will consider it a threat and focus all their resources on it even if the ships keep getting owned like crazy. I assume that this an exploit in current game AI, but can help you deal with nasty situations.
So overall, if you dominate the map economically and militarily you should have no problem leaving some of your important outposts semi-exposed. Maybe if you laid out your specific issues we can help better.