Apologies if this post comes over as teaching you to suck eggs, or as too technical...
For the router replacement - for a home network - I think it could be done.
However your ISP (internet service provider) will be of no help in troubleshooting any issues that you have.
As the people on the Helpdesk will have a script to go through and one of the first things they will say is "Switch off the router (that we supplied to you) and switch it back on" Followed by "What LED's are showing on the router (that we supplied to you)?"
If you've got some industrial strength Cisco or HP Procurve router, you're not going to get very far into the helpdesk's script. Unless you lie to them.
From their point of view they want all of their millions of customers to have the same router, with the same config on it (done by them). It makes support so much easier.
If you can configure routers you should be fine. If you can't configure them, you're stuffed, because you'd at least as a minimum need to configure your ISP specific WAN (Wide Area Network - your connection to the internet) credentials.
The tightest bottleneck will almost certainly be the speed from your local internet distrubution box to your internal wall socket.
As an example, the speed for me is of the order of 70 mbps (mega bits per second) download and a measily 20 mbps upload speed. That's with a fibre to the cabinet connection. This time 2 years ago I had a much slower connection than that.
Once inside your house, 18 year old switches will do 100 mbps in both directions.
More modern switches will do 1000 mbps - commonly known as gbps (gigabits per second)
And if you get into fibre optics you can get speeds of 10 gbps or higher - which starts to get a bit daft because hard drives then start to become the bottleneck.
Gigabit switches (max speed of 1000 mbps) are pleasantly affordable on ebay - if you buy used. The industrial strength Cisco and HP Procurve ones are good. Reliable. Well engineered. Most of them have cooling fans. Which is fine if your switch will live in the basement. If it will live in your listening room, get a fanless switch.
In reality, for a home network, I bet I could swap out a 100 mbps with a 1 or even 10 gbps switch randomly and you wouldn't notice the difference.
I've not seen anyone use fibre optics for their home network for hi-fi use.
Theoretically this should be the bees knees.
Last time I looked I estimated that it'd cost about £300 to connect up your main home devices with fibre. That's going 2nd hand. With a risk that the fibre NIC (Network Interface Card) drivers might not work (well) with your PC. Because used fibre NIC's would typically have been used in servers in data centres. Like Google's servers. Real mega industrial strength computing.
Fanned Cisco and HP Procurve copper switches can be bought for about £20 off ebay. They would have been in the region of £800+ when new. Fanless copper switches cost a bit more, about £50. The lower electrical consumption of fanless is nice - for something that's on 24/7/365. Although fanned switches have longer MTBF (mean time between failure).
As for cables, Cat 8 can be bought from Farnells for £15.42 for a 5 metre length.
https://uk.farnell.com/c/cable-wire-cab ... egory=cat8
Or you could look out for reels of cat 8 on ebay. As well as deals on the cat 8 plugs - which so far I've not been able to find at a reasonable price. Making your own cables is easy. and makes sense if you want something that's exactly the right length.
With Cisco and HP Procurve switches you can just plug them in and they should work with the default factory settings. However I do wonder if putting a half decent config on them would make them sound any better? If nothing else, a config should get a device working on the network quicker when it is plugged in - due to techie stuff like spanning tree portfast settings.
For Cisco AP's (access points - to extend your wireless network) they DO need to be configured to work.
Cat 5e cables are fine from a home network speed point of view. From a sound quality point of view, try different cables and see what you think.
I have so many Cat 5e cables that I give them away free to friends and clients.
It's also worth noting that there may be a high proportion of Cat 5e cables out there that are fake. They can get away with fake cables because they still work fine from a computing point of view - probably because most connections get nowhere near the specified limits of 100 metres and 1 gbps speed.