I do wish people would stop recommending a pi for a router. Its wifi is weak due to internal aerials and its limited to 8 clients on its wifi.
A router should have DUAL LAN ports to give you dedicated WAN and LAN.
Use the R4S instead with a switch and dedicated AP. You will have far less issues.
(edit) - @daydaystech Also that is a horrible async. as moeller says you will use most of your upstream managing that downstream. You definitely require QOS setting up internally to manage your upstream with 8 people sucking on that pipe. All it will take is one person running a torrent to murder the rest of your gaming.
having gone and had a read up on this. I have a few questions. Firstly it is way more powerful than the R4S or a pi. Why replace the default firmware that Asus provides? They have gaming QOS built in and preconfigured. Seems bit odd to replace that unless you are wishing to manage your own QOS settings?
I hope you are using a 2.5gb switch to backbone the router yes? or relying on wifi for other clients? or using the 1gb ports on back?
What you could do to take some of the load of your GT-AX6000 is to turn off the wifi on it and serve wireless via other APs. That would enable the cpu to only be used for routing instead of also being used for wifi if you are concerned with the cpu load. However the load should be shared across all its cpus. Have you checked that the merlin firmware uses irqbalance or other cpu sharing methods?