There's a reference in a TNG episode to a Tholian raid and how Riker's father was involved somehow in the aftermath. Strikes me, that like the Borg... there was never much of a war as we know it. Just a series of skirmishes near their border and unease whenever they show up. By DS9, there's talk of an Ambassador but it's just lip service paid to TOS really, since we never actually saw it. Probably would've been too much like Babylon 5's Kosh, floating around with the aid of an EV suit in our environment. There's a moment in Future Tense when they're about to board the NX-01 to retrieve that timeship... I assume they're well equipped to move around in our atmosphere.
The interesting thing to me, in terms of the Tholians is their involvement in the TCW. I guess like the Suliban or the Xindi, they had their own "Deep Throat" spilling beans about the future... or not, depending how it suited some unknown agenda. Too bad it took the Mirror Universe for us to actually see one in the flesh... er, crystal? There was really nothing in canon that precluded the Tholians from being a big bad on Enterprise.