Just wanted to add a positive data point to this discussion, as I read it a few days ago before upgrading my plan from 1000/100 to 2000/200 (I also have the AX86U) and feeling a bit discouraged.
The setup I have is the ISP’s ONT (provided for free) 2.5GbE port connected to the AX86U 2.5GbE WAN, and then I connected both LAN 1 and LAN 2 on the Asus to a 6 port switch I have (all ports are 2.5GbE) and enabled link aggregation on the Asus and LACP on the two ports on the switch.
Now I effectively have 4 ports on the switch that can use 2Gbit speed, and I consistently get 1900/200 when checking on speedtest.net with a wired mini PC that has 2.5GbE ethernet connection.
When I do a speed test using the internal Asus page (to the exact same server) I never pass 1200 download, so it really is just limited by the CPU when testing, but not when routing.
So my conclusion is, if you care about a single client using 2Gbit speed with the AX86U, this seems like the only way.