i see the z390-i also has the intel i219-v lan just like on my xi hero.that lan card can cause lots of dpc latency spikes due to the interrupt moderation in the advanced driver settings.you can try a newer driver or also check in latencymon to see if...
seems it was more of a side grade or downgrade really.as for rtt values you can try setting those according to ryzen calculators suggestions and it might help a little but i wouldnt get my hopes up.try stuff like disabling vrm spread spectrum and bcl...