4 phases with ic doubler aren't "real" 8 phases either.
That said, any of the ROG boards will oc a 9900k to 5,2 without running too hot and better than the z370 counterparts.
But... the Aorus z390s have overengineered VRM (particularly the Master and the Extreme) and are priced very well. Memory overclocking is not the best, though and the bios is less friendly than Asus and Asrock. Pick your poison.
new rig 2019:
Intel 9900k >5ghz | Asus Maximus XI Gene | Eisbaer 360 LT (custom loop in progress) | bequiet! Dark Base 700 white edition | Team group Nighthawk 16GB 4000mhz | MSI RTX 2070 Armor | Storage: Intel Optane 900p 280GB + Samsung 970 evo plus 1TB + WD Red 3TB | Corsair HX850i | Benq BL3200PT 1440p | 1Gbit ftth |