JohnnyCamelot has three usable network interfaces

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PS C:\WINDOWS\system32>  Get-NetAdapter

Name                      InterfaceDescription                    ifIndex Status       MacAddress             LinkSpeed
----                      --------------------                    ------- ------       ----------             ---------
Ethernet                  Realtek PCIe GbE Family Controller           24 Up           FC-5C-EE-02-B8-70         1 Gbps
Bluetooth Network Conn... Bluetooth Device (Personal Area Netw...      16 Not Present  B8-1E-A4-7C-32-92          0 bps
Ethernet 2                Realtek USB GbE Family Controller            15 Up           A0-CE-C8-56-E2-4C         1 Gbps
Wi-Fi                     Realtek 8822CE Wireless LAN 802.11ac...       8 Up           B8-1E-A4-7C-32-91     866.7 Mbps

JohnnyCamelot has three usable network interfaces.

Adapters

Ethernet     → Realtek PCIe GbE
Ethernet 2   → Realtek USB GbE
Wi-Fi        → Realtek 8822CE

So the machine has:

NIC 1  built-in Ethernet (PCIe)
NIC 2  USB Ethernet adapter
NIC 3  Wi-Fi

Both Ethernet ports are running at:

1 Gbps

which is ideal.

This is actually perfect for a gateway machine

You can now cleanly separate networks like this:

Internet
   │
Koodo router
   │
Ethernet
   │
JohnnyCamelot
   │
Ethernet 2
   │
Switch
 ├─ Camelot
 └─ Frodo

Meaning:

Ethernet    = WAN (internet side)
Ethernet 2  = LAN (internal network)

Wi-Fi can be:

backup
or disabled

Why this is better than earlier ideas

You now have a proper two-NIC gateway design:

WAN NIC
LAN NIC

which is exactly what routers/firewalls normally have.

Your earlier reaction about USB NICs now makes sense

You said it felt weird — but in this case it worked out perfectly:

PCI NIC  → WAN
USB NIC  → LAN

Many small firewall appliances actually do exactly this.

Next step (just observation)

Let's identify which interface is connected to the router.

Run:

Get-NetIPConfiguration

Look for which adapter has:

DefaultGateway

That one is currently the internet-facing interface.

Why I'm interested in that

We want to decide something simple:

Which NIC becomes WAN
Which NIC becomes LAN

Once we know that, we can wire the topology cleanly.

One more thing worth noting

Your hardware is actually quite ideal for this role:

Realtek PCIe GbE
Realtek USB GbE

Both are gigabit, so the gateway will not be bottlenecked.

Paste the output of:

Get-NetIPConfiguration

and we’ll map the interfaces and decide the clean layout.

Perfect — this shows exactly what happened.

When you enabled Internet Connection Sharing (ICS), Windows automatically reconfigured the LAN interface.

It changed:

Camelot interface → 192.168.137.1

That is normal ICS behavior. Windows always uses:

192.168.137.0/24

for the internal network when ICS is enabled.

So JohnnyCamelot is now acting as:

Gateway: 192.168.137.1
LAN:     192.168.137.x

What this means

Your topology right now is:

Internet
   │
Koodo router (192.168.1.x)
   │
Wi-Fi
   │
JohnnyCamelot
   │
Camelot interface
192.168.137.1
   │
LAN devices
192.168.137.x

Windows also automatically:

runs NAT
runs DHCP
assigns addresses to LAN machines

Important consequence

ICS does not allow arbitrary LAN subnets.

So your original design:

10.20.30.x

cannot coexist with ICS unless you:

disable ICS and use Windows routing instead, or
accept the ICS subnet.

Easiest path forward

For now, accept:

192.168.137.x

Then on Camelot set:

IP:      192.168.137.2
Gateway: 192.168.137.1

Then test:

ping 192.168.137.1
ping 8.8.8.8

If those work, your gateway is functioning.

Key point

Nothing is broken. Windows simply enforced its ICS default LAN network.

If later you want to restore the 10.20.30.x architecture, we can switch from ICS to Windows routing (RRAS-style) which allows custom subnets.