so to address all your points, p5s will hopefully be announced for WW soon, but it strange that we haven't heard anything, but the release periods between JP and WW for p5 was just under 7 months, and exactly 5 months for royal, so i'm not surprised it hasn't come out yet.
the game is a sequel to base p5, as it started development right around the JP release date of p5 (so obviously the royal story wouldn't be close to finished when they were writing the story for scramble). all in all, if you don't do the new palace in royal and end the game after the fight with yaldabaoth, the story is almost identical, just with one or two characters missing. the story doesn't change heavily with the inclusion of kasumi, maruki and jose until the new royal content, so that stuff doesn't matter. the timeline is a bit strange if a new player started with royal and got the new true end, but oh well sucks for them. i wouldn't be surprised if they added kasumi as a dlc character at some point in the future, but who knows.
tl;dr sequel to base p5 but the story's not much different even with kasumi and maruki
