I have no idea how this works, but I hope Talamasca being canceled doesn't mean anything negative for IWTV. Negative only in the sense that maybe them canceling one show in the franchise means they're more likely to cancel other shows in the franchise? Though they did say this:
The Talamasca has a storied place within the Anne Rice Immortal Universe, and we expect to see at least some of these characters, and the organization itself, in future expressions of the franchise
which makes it sound like they're possibly going to create more original shows in that universe, so they might just be canceling one that didn't work for them. And since it's based on original material, not books, it's a lot easier to cancel. And I can see why, honestly. I didn't love it, and it didn't get really good reviews. And I feel bad for him, maybe he's great in other things, but the lead actor especially did not get particularly good reviews, so that's hard. And it seems like AMC loves that IWTV is such a critical favorite, and then Mayfair at least has viewers (though they did bring in a second showrunner with more of a prestige background, seemingly trying to make it more acclaimed).
So maybe this has nothing to do with IWTV's chances of getting another season or two. And I am more relaxed now that we at least got season 3 and TVL, though I would be really disappointed if we didn't get season 4, of course. But I think it's likely they're really planning on that, and canceling IWTV would be a much bigger deal than canceling Talamasca. I kind of doubt they're planning more than 5 seasons of IWTV anyway, based on what Rolin has said. Maybe I could see 6 but I think 5 might be more likely.
It's a shame because I do think the concept of a Talamasca show has legs, but I agree that what they did with it didn't really work, so I'm not terribly surprised it's been cancelled. I do feel a little bad for the lead actor too given they really hung the season on his shoulders and I just don't think the writing was strong enough to lift his performance, nor his performance strong enough to elevate the material, although I will say I feel a little less bad for him since finding out he's Jane Turner's son (she's literally an Australian comedy icon, he'll be fine, haha).
I don't think it really has any bearings on IWTV though. From Rolin saying that they're splitting TVL in two and this season being seven eps, I suspect that AMC has given them a 15-episode order to adapt the book like they did with IWTV. That would mean that s4 is already more or less greenlit, and I can't imagine that AMC wouldn't want Rolin's version of Queen of the Damned, not just because they're clearly investing a lot in their relationship with him, but that source material's iconic enough that it'll be buzzy no matter how it shakes out.
With IWTV and Mayfair Witches as the tentpoles, there's really so much storyworld to explore, and it seems like they have a lot in development. If they feel Talamasca didn't land as they wanted it to, it makes sense that they'd shift focus.