No need for cheats,
take the bonus tiles as they are, there are always manu & research bonus tiles on most planets
you can lower difficulty (cakewalk is still hard enough against the DL's)
many tips can be found on these forums, the main ones being:
*rushing is the key in most missions;
given the time, the DL's will build up a very tough fleet
*you can always review the victory conditions during the mission
just follow your briefing, completing the objectives wil result in victory
no need to wipe everyone out, if you only need to take 1 planet
*play the alternatives if you can't hack the first 5 missions, they will get you along just the same..
*DL ships are like eggshells; as long as you first strike them with decent force, they will crack
*leave your planets unprotected (no ships in orbit) to prevent being swarmed by DL's
this will draw in unescorted DL transports that you can easily take out,
if one gets through, with enough pop you won't get invaded
if you do lose a planet, retake it & meanwhile get a great weapon tech