Your take is wrong. S4 showed a potential back door pilot to 32nd Century SFA, with Tilly teaching at the academy. There's definitely potential there for Mary Wiseman (or anyone else from the original crew) to lead a new show. There are plenty of supporting characters from S3-4 (and soon 5) to explore that time period still with something fresh. Discovery may not have originally been planned for 5 seasons but it's still a successful run. 7 years is a long time for an actor to be tied to a particular project. I imagine that a 6th and final season was always a question mark and 7 was probably never in the cards.
My speculation is that P+ will spin off new shows from both SNW and DSC so you have two concurrent live action shows per season. Titan is a possibility but even if the show gets ordered tomorrow, there's still two years plus of casting, writing and shooting before we see anything on screen, and I doubt that any decision will be made until Picard finishes its run.
The ability to tap legacy trek actors for guest or recurring roles suggests to me that they may continue to shoot in LA instead of tearing down the sets or moving everything to Toronto.