Well, it was a nice idea while it lasted, but the grand plan to shoot both of the next two Mission: Impossible movies in one big block is coming to an end, at least according to Deadline. And it's not the Covid delays or the trickier shooting situation that is responsible – it's Tom Cruise's availability.
Mission: Impossible 7 has been filming in various global locations since re-starting, including a recent stint in Abu Dhabi (see writer/director Christopher McQuarrie's Instagram post below) and a return to London for the finishing touches. Paramount reportedly needs Cruise to be ready to promote Top Gun: Maverick's planned summer release, which will throw a spanner in the works of future shooting. Once the Top Gun sequel is out, he can return to film the eighth Mission.
Release-wise, Mission: Impossible 7 is on track for 19 November this year, while the eighth movie has a 4 November 2022 release on the books, so a brief production delay shouldn't be an issue.