@Brandburner 

Deco P9 can use powerline backhaul link and will prefer it. You can't control how P9s are connected to each other.

Deco M9 plus does not have powerline backhaul. With new M9 Plus firmware (currently in beta) you can configure how they connect to each other and to the rest of your Deco mesh. Which is handy when you have large Deco mesh (and you already do). More about that firmware: https://community.tp-link.com/en/home/forum/topic/574954

The answer will depend on the following: do you have good clean from noise electrical wiring where you plan to place new Satellite Decos? Because, if you do, P9 makes sense. If you don't, P9 may deliver worse Internet speeds than M9 Plus - then, go with M9 Plus..

You can test that now, with Deco units you already have. Place your P9s where new units are supposed to be and see if they deliver good speeds. Then, instead of P9s place your M9 Plus Satellites there and compare results.

  @Alexandre. Thanks so much. Yes I think I will try switching the layout of my existing setup first.

I know thee official advice is to connect M9 Plus ahead of the P9, but doing this seems to mean the first P9 only receives a (lossy) wireless connection from the M9 before the powerline magic happens.

Should I expect any issues if a P9 became the main deco?

  @Brandburner 

There should be no issues in having P9 as Main Deco in your Deco mesh. Here is the process to do the change without losing Deco mesh configuration settings: Changing the Main Unit for your Mesh Network

You have challenging set of Deco units in your mesh. M9 Plus works the best with other M9 Plus, P9 works the best with other P9. M9 Plus and P9 will coexist together, but you are going to have challenging time finding optimal mix and units placement.

If your house electric wiring is good and P9 provides acceptable Internet speeds through powerline, try building Deco mesh with P9s as Main Deco and Satellites in all critical places first.

Then, if you don't have enough WiFi reach from Satellite P9 to some corner of the house, deploy M9 Plus you already have in between that Satellite P9 and that corner, and configure M9 Plus to connect to that Satellite P9 specifically. Meaning, M9 Plus will become range extenders for P9s, where it is necessary.

Once again, that only makes sense if P9s do well with your house electric wiring, and that you will have to test yourself.

Also, if that works, your next purchase of Deco units should be P9s.