IIRC, the original plan was to make nine films, with the first three being in the middle, the second three being the meta-prequel, and the last three being the meta-sequel, so this would be following the original plan.
In any case, those of us who were at "Yo-Hi" (Nile C. Kinnick HS, now at the Yokosuka Naval Base but in Yokohama back in the day), when Mark Hamill was starring in the school plays, will be interested to see how MH fits in the role of Old Mentor; he will have to play it very differently from Sir Gielgud.
I’ve never really bought the “9 movie plan”. Too much of the story is clearly not done with a long term plan (like Luke and Leia’s attempted relationship in the first movie and then they’re siblings in the third).
I was 11 in 1977 when the first Star Wars came out. I remember my school bus driver told us Lucas’ plan of the middle three then the first three and of course the last three as you pointed out. I think the original plan was to release movies 1 thru 3 in 1986, 1989 and 1992 with Episodes 7 thru 9 in 1995, 1998 and 2001. I wonder how an Episode 1 filmed in 1986 would have looked, probably not as much CGI although in my humble opinion Val Kilmer would have made a good, young Obi Wan.