Oh boi… What an experience listening to this one!
Definitely my favorite of this Series. I am not necessarily sure how I’d rate it, it’s very good and made me really hooked into this Series compared to the first three Entries. I do have a lot of thoughts and I do agree with a lot of what has already been said, so forgive me if I repeat anything here:

I think Louise gives her best Performance in this Series here so far at least, it’s marvelous and just great. There are a lot of Nuances in her Voice Performance, be it shifting to a softer tone or a harsher one, it’s clear that Louise knows her Stuff and does so well expression every Emotion simply with her Voice. This is Voice Acting!
Of course all the others do give a great Performance as well, I mean how can you not love Lalla Ward as Romana? Fisher as well gives a great Performance as Sissy, even if at times I had a hard time getting used to a different Character having such a familiar Voice, but of course no disrespect meant at Fisher. With that Character, I can only echo what others have said. I think it’s a good portrayal and fits the Theme of the Story very well, even if I sightly wish that it could have been explored a bit further. Then again, what we have I think is already pretty good.
especially that one particular Conversation she has with Leela has a lot to offer, both for the Characters, and for the Themes.
I must admit I remember accidentally spoiling myself on the Twist, but it probably has been such a long while, so I kinda forgot it, so it did surprise me. It’s a very clever way of going about it and uses the Nature of the Timelords so well, and of course is a pretty satisfying Payoff.

While a lot is going, this Finale does not forget its Character and slows down when it needs to, and I think those Moments are my favorites and work so well when compared to the at times hectic pace of this Audio.
I think I’d have to sit on this one for a bit before I can properly say where I would place it or what I would rate it. But it’s very good for sure.