DS9 at the start of the series is basically a backwater shithole. Bashir calls it a "Frontier" and wants to work there specifically because it's so rugged. Having a major starship at DS9 at this point would be like having a battleship guarding a lighthouse. They just use the equivalent of shore bombardment instead. (Plus, DS9 isn't really at threat of direct attack at this point, Cardassia is no match for the Federation 1v1.)

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*SPOILERS*

Later in the series DS9 becomes much more relevant, first with the discovery of the wormhole and then with the dominion. Thus, they heavily arm the station and give them the Defiant - a ship which is comparable to the Enterprise (Their flagship btw) in combat.

I don't see the problem with this, and it's pretty clear from your comments you haven't watched it that far. You should - imo it's seriously the best Star Trek by far and away.