Obviously being the first port of call from the wormhole DS9 was the first line of defence against the Dominion. It got some decent weapon upgrades but since we know Orbital Weapons Platforms exist, how about a few of those? Chief O'brien had no choice but to use the old cardassian computer systems because he had a small team and no resources but since this is now the most important strategic asset why wasn't the station swarming with dozens of the best techs and engineers ripping out old systems and replacing with state of the art tech? Maybe a soverign class starship assigned to the station might strengthen starfleets hand a little? From an in universe perspective it just seemed a bit slapdash considering the strategic importance.
Supposedly, starships don't help much in defending starbases - it's vice versa. Or at least the Defiant never tried to interfere when DS9 was swatting enemy fleets out of the sky. Said swatting was rather different from the early days when the station couldn't ward off three Cardassian rust buckets or reliably lock a tractor beam on fleeing riffraff. I'd say DS9 got exactly the sort of upgrade you depict, because that's what we saw in action. As for multiple platforms, probably not - the wormhole was a compact point to defend. And extending the defense across the star system would make little sense, as losing the wormhole would mean losing the game altogether, and nobody from the Alpha side would be attacking anyway (or so they thought before "Way of the Warrior", and again before "By Inferno's Light"/"A Call to Arms", and afterwards they were more or less right). Except for sabotage runs against the wormhole itself, and a systemwide array of defense platforms wouldn't stop individual cloaked saboteurs (as nothing stopped the Romulans in "Visionary", say). Bajor would no doubt hotly object to UFP fortifications in the system anyway; they'd always be either too much or too little. Timo Saloniemi
The more high tech equipment they put on DS9, they more trouble they'd have ripping it out when, not if, they had to abandon the station. They didn't want to give the enemy that many Christmas presents, I suspect, and unlike a warship, this had many civilians and was Bajoran property. They could not scuttle her if they felt they were about to lose their asset.
Not to mention, but until Sisko yelled at all the admirals about where the "hottest" piece of real estate was in the Alpha Quadrant during the war, everyone else was concerned about reinforcing Earth, Vulcan, Andor, Quo'nos or the other core worlds.
Well, the one time DS9 really was "hot", no enemy was going to attack it, because it was already in enemy hands! This also mightily inconvenienced any attempts at upgunning it. Once off those hands, it apparently ceased to be "hot". Or at least no enemy tried to attack it again. Which is understandable, because the only claim to value there was the wormhole, and forces greater than the guns of DS9 were in charge of that wormhole most of the time... Timo Saloniemi
The whole point of 'Call to Arms' is that they know they can't hold Deep Space Nine against a determined Dominion assault. It was a stone's throw from Cardassia Prime. Assigning more ships would just have meant losing more ships for nothing. The minefield was the key, once that was in place DS9 had little value for Starfleet, especially with the Bajoran government taking a neutral stance. Meanwhile the main Klingon-Federation force was busy destroying Cardassian shipyards - a much more important target which possibly allowed the allies to hold out against the initial Dominion assault. What's maybe more curious is that the Dominion never made another attempt to retake or destroy the station, given the wormhole was the key to their success. They didn't know what happened to their invasion fleet when it disappeared. Only crazy Dukat had some theory about wormhole aliens, which was indulged.
Then again, why try and take a worthless wormhole? Holding onto it seemed difficult enough the first time around; retaking it should only be done after they find a surefire means of forcing it open a second time. They could then be certain of holding it, with the help of further ships immediately pouring in - or at least would not be on a schedule, and thus could receive their new ships, withdraw at their leisure before Starfleet reinforcements arrive, and then strike again when Starfleet is in the wrong place. Timo Saloniemi