So the Sagecom router/modem combo is capable of 1.5Gbps on one port, where you plug in the internet (this is called the WAN port). All the other ports (the LAN ports, where you hook up your wired devices) are only capable of 1Gbps.
This means, that if you hook up a wired PC/Playstation (with an ethernet cable) to the Sagecom device, that wired devices will not be able to exceed 1Gbps on that cable, but all of the devices together combined (with the wi-fi devices included) could do, so they could generate 1,5Gbps data usage together at the same time (note: it is rare when that happens with normal everyday "family" usage)
With the 1Gbps capable Asus routers, that could not happen because every port is 1Gbps (including the WAN port).
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The Wi-Fi6 wireless capability of the Sagecom device is also a newer technology compared to the Asus routers you have, which are only Wi-Fi5 standard capable, but it is important to understand that all of that is only on paper. There is more than just supported standards (Wi-Fi6 vs Wi-Fi5) when it comes wireless networking. I tried to search for info about what kind of antennas your Sagecom unit has built in (how many devices can it talk to at the same time and at what speeds, etc,), but I found inconclusive results (For example. it looks like Bell in the States handles out this very same Sagecom unit with 2x2 Wi-Fi5 capability and 1Gbps ports only).
That is why I said that you would need to test things with both devices, because with that many wireless clients (STAs), you might find that the AC86u actually provides better coverage and speeds, despite its older technology.
(I would not consider the AC68u a viable alternative against the Sagecom when you already have the AC86u at hand, that is a much better router, but don't worry because as an AiMesh node, the AC68u can be still very usefull).
My mind says that you should listen to
sfx2000 and use the Sagecom, but my Merlin infested heart compels me to say you should try the AC86u (I would definitely sacrifice the 1.5Gbps speed to use the AC86u, but that is stupid me, no cure for that)
Sorry if this all sounds too complicated, perhaps somebody else with better English can explain it better to you.
ps.: Just one advice: in general, it is always better to hook up devices with an ethernet cable instead of using Wi-Fi, this is especially true with gaming consoles or PCs).
Good luck.