R6700v3 unable to support 600Mbps - capped at 300Mbps

[User987654]

R6700v3 purchased in 2019.

Recently experienced terrible streaming experience with 600 Mbps package. Direct ethernet connection to modem clocks at 700 Mbps.

Disconnected modem and router and all cables for 1 minute.
Updated firmware to v1.0.4.126_10.0.102.
Got new Modem CM2000 (2.5Gbps).
Factory Reset Router.
Enabled IPV6.
Disabled: Access control, traffic monitoring, parental controls, QoS

Direct ethernet connection to router clocks at 200-300 Mbps. Same with 5G-wifi.

Time to switch?