Could have been a skeleton crew onboard for all we knew, the dreadnought itself presented just enough value to see whether Bull would chose the Qun or his men, after all, he was supposedly one of their top agents?
It's not like we've not seen the Qunari do these sorts of excercises before, in DA2 the Arishok leaks info about the gaatlok (actually saar-qamek) to see if anyone will attempt to steal it, purposefully sacrificing the token amount of guards he assigns to guard it in order to make it look valuable enough? In that case, he deemed the knowledge of the conspirators plotting against them more valuable than a few guards lives? But, if the conspirators hadn't taken the bait, then they weren't enough of a threat to worry about and his men would still be alive, thus he loses nothing?
While it might seem a waste to send a dreadnought on a potentially suicidal mission, the probable reasoning behind doing it that way was because the desired outcome for them was to walk away with both the and Bull's loyalty intact... which means their gamble would have cost them nothing and instead gained them an alliance with the Inquisition? To drag out that (tired) old meme, "Heavy risk, but the priiize".
Furthermore, the Qunari's value on the acceptable lose of various assets seem to be flexible when it comes to the success of the mission?
In DA2, the Arishok was not angry when Hawke admitted to being forced to kill the Arvaraad (he was actually impressed with both the skill it took and Hawke's honesty), because as far as he was concerned, the mission to recapture and kill the Saarebas was all that mattered and it had been accomplished, while those men were unfortunately collateral damage taken while in service to the greater goal?
The Qunari have an extremely alien way of thinking and while they don't usually act without careful consideration, they do seem to be in favour of taking calculated risks if they deem something potentially warrants such gambles to be taken? Losing a gamble however as badly as they can in DAI, is probably why they immediately cut off any prospect of an Alliance and declare Bull Tal-Vashoth?
That explanation would be a lot more convincing if there were anything in the game that actually implied that the writers meant for it to be true. Which there isn't.
Gatt outright states that the mission was partially laid on to test the Iron Bull's reliability and to see how worthwhile a close relationship with the Inquisition would be. But "partially" is a long way from "wholly", and the game offers plenty of other explanations for the mission without ever directly implying that they are meant to be disregarded. (There's more than one instance that indicates that they
aren't meant to be disregarded. Asking Bull about the plan gets you an interesting response: he admits he's uncomfortable about the way the plan could go, but ascribes that to the general character of the mission - "too many ways for things to go wrong" - rather than an unnecessarily idiotic plan.) And the way in which the mission went pear-shaped had far too much to do with things outside Qunari control to be particularly plausible as a means of creating the Sophie's Choice for Bull.
It's far easier for me to accept that the writers weren't able to come up with a particularly good plan for the Qunari operation - especially since it's been repeatedly demonstrated that "realistic or plausible warfare" is not something that is in their collective wheelhouse - than it is for me to accept that the Qunari plan was intentionally bad so as to create a moral dilemma on the Bull's part.
In my experience, trying to fix writer errors on one's own is an excellent way to get oneself frustrated without really achieving anything, because invariably the writers themselves will contradict one's headcanon anyway.