[Unknown_Enemy]

The problem is certainly not in the maintenance costs. The problem is the long build times.

You want the AI to pump out tons of cheap colonizers, instead of tying up a valuable spaceyard for many turns just to have the finished ship dismantle to make a colony.

Indeed.
The borg need a cheap coloniser build very quickly. Thus the 300t needed for a colonization tech on their other ships can be used for weapon/armor/shields. As it is on Rage's ships.

Your remarks on the borg's weapons were acurate, but it's just that the Rage were bearing more wepons/armor/shield per ships.

When the Rage start using warship to colonize, they use...light cruisers.