Seems a little impractical to have it minimally armed, unless she were to always have several guardships permanently assigned to her.
I would think that all Starfleet ships would need a decent level of tactical systems for defence (just look at the number of phaser arrays a Nova-Class has).
Well, yes. Why not put several guardships around her, just like several Destroyers that guard a US Aircraft carrier?
It is an expedition to a far-far away galaxy; Far from even the nearest Federation Outpost in Milky Way Galaxy. So basically it should be self sufficient. And what kind of starship that can handle everything? Forget Voyager. We are not creating Voyager's clone.
So the Enterprise would be a kind of mothership. the central command for all Federation activity in this new galaxy. It is not a colony ship of course, but it has a lot of system that can handle the need of long time mission. The idea is that the fleet capable to harvest dillitium crystal / other important mineral themselves. For combat department, we have already have other smaller starships. Like the one that I have mentioned in my idea.
In this idea, Enterprise would act as semi starbase; while the smaller starships will do exploring. The setting will be inside the Enterprise, in the smaller exploration starship, and maybe in a mining ship.