For me, the perfect level is 8, and I have Rebel bases at level 7, 8, 9 and 11, and Imperial bases at level 7 and 11, and I can guarantee that the best base for playing Conflicts and Events is my Rebel HQ 8.
Now, to play Wars, the best base is my HQ 9, for the advantages described above - LAAT/IDT traps, Oppressor, 4 shields, access to 6 planets.
As maxed HQ 9 you will be combined with Prestige almost all the time, while as maxed HQ 8 you will be combined with HQ 9 and weak 10s - of course the level of rivals will depend on the value of your base score and number of medals.
Either way, PvP dies when you get to 10 and get buried at 11. If HQ 8 is boring, go to 9 and stay there because you will still be able to dispute conflicts and advance in events without having to go through nightmares like deploy 412 vehicles, destroy 270 turrets or get 790 EP.
SWC Windows survivor.
Imperial HQ 11 & Rebel HQ 11 on vacation.