Summoning rituals are a weird element in general. Call of Cthulhu adventures have long suffered from the problem that, if the players win, they get an anticlimax as the bad guy never arrives. The adventure just kind of ends without the dramatic big note. However, if the bad guy arrives, everyone dies. D&D lets you have them fight the bad guy - different power scale - but it doesn't get away from the anticlimatic nature of actually stopping the ritual.
This leaves the DM with two options: let the players stop the summoning ritual, and thus have no boss fight to end the adventure with, or have the thing arrive anyway, in which case the players might well deduce that there is no time pressure (can't stop it, regardless of how fast you run) so they might as well take things nice and slow. Neither one feels perfect, unless you manage to have a sufficiently impressive 'high priest' equivalent conducting the ritual, against whom the party can have their boss fight.
The Dragon Masks are kind of this problem writ large. Do you want the players to be able to stop Tiamat from arriving? If so, they can get the masks. If, however, you want them to be able to fight the creature which, remember, is on the cover of the book, then either they cannot get the masks, or the masks are irrelevant for the ritual. I'm kind of leaning towards the latter; the masks are very powerful artefacts that Tiamat has given to the cult and which will help them conduct the ritual better, but they are not actually required. In other words, each mask they find will reduce her stats, as outlined in the back of Rise, albeit without much support in the adventure for the idea.
I might also outline to the players that once the final battle starts, they get X amount of time until Tiamat gets summoned - X being equal to the fastest possible route, so they cannot stop the ritual - but that there is also Y and Z amount of time until the ritual progresses to the point that they can make up for the missing masks. This way I can get time pressure, while also getting my boss fight, and the players' accomplishment in getting the masks has an appreciable part in the finale.