I saw your post on Reddit. You won't get much input here from users. There are only 3-4 of us here(besides the moderators).
With that said, most problems are related to signal and not hardware. AFAIK all DOCSIS 3.1 modems support up to 1Gbps(hyperthetically) while actual speed maxes around 940Mbps because of overhead. You can squeak out a bit more by getting a D3.1 modem with a 2.5Gbps ethernet port like the S33, but that won't help a signal issue. See here for a full list.
Can you post your signal levels? See here.
As for your router, that model is EoL as per here, so even if just for security reasons, probably a good idea to replace. I have the RT-AX86u and love it. They have a Pro model though which is better and I think the same price now. The gl-inet FLint 2 router is also a good choice. You won't get gigabit over wifi though unless you invest in a Wifi 6e or above and you have devices that support it.
