DT have always counted anniversaries from the moment they started, that is 1985. The 20th anniversary tour was in 2005 and their short 30th anniv. tour was in 2015. Whether they make it to 50 (2035) is a completely different story.
Right now, it's all about what are they going to do for their 40th, which I'd love to see celebrated properly and not like they did for the 30th.
If they release a new album in early 2024, they could type that whole year for it and then head into 2025 with a tour fully focused on the celebration, not an album promotion cycle.
As for MP being part of it or not, I'd welcome a one off "big" show with him, but more than that would be a huge warning sign for MM, IMO. However, I have to say the WDADR version of Metropolis with Charlie and Derek is my all time favorite live version of the song and would love to see something similar happen with the current lineup + MP and Derek.