I have one completed PT where my PC is a Rift mage, so no problem not having Solas along for the DLC. In my other PT, though, the one that's a better fit for the DLC RP-wise (a dragon hunter), there's no way I'd go into a challenging quest without Solas. So I'm forced to do the DLC before the main quest is resolved.
Just based on the implied pacing of the main quest story line, it's probably best done before What Pride Had Wrought, right? The Temple of Mythal onwards is all one sequence to me. I mean, it would be kind of silly for Cullen to be worried about the forces taking too much time to get back to Haven, so instead, the Inquisitor goes off to DLC land for a vacation.