You can do that, but it's a bit more complicated. You'd end up with a Local Area Network/Storage Area Network (LAN/SAN) setup. The big thing you'd need to remember is that you need to do different IP ranges for both. To keep it simple I'd use something like a subnet of 10.0.0.0/8 for the LAN (going through the router with access to the internet) and a subnet of 192.168.0.0/16 for the SAN (for the connection between the server and the gaming PC). You also wouldn't have DHCP (automatic IP address assignment) on the SAN, so you'll have to set static IP addresses there and keep track of things.
You should be able to use a standard 3M 100G DAC between the server and the gaming PC to accomplish this. For other devices, you can put the server on the LAN as well as the SAN; just make sure you use the 192.168.0.0/16-range IP address to connect to the server from the gaming PC to use the SAN connection and access the higher speeds.
However, you'll still have some speed penalty for local network traffic that goes though the LAN. A router is designed to to actually route traffic between networks. In your case, it would do the work of Network Address Translation (NAT), among other things, allowing you to have multiple IPv4 addresses on your private network and connect those IPv4 addresses to the broader internet. It also lets you move traffic between multiple private networks; say you want to keep your guests on a 10.10.2.0/24 network while all your sensitive equipment is on a 10.10.1.0/24 network but you still want your guests to be able to access a printer on the sensitive network. It's also typically your DHCP server. All those tasks would be the role of the MikroTik CCR2004-1G-12S+2XS in this setup.
However, enterprise routers (like the ones you're looking at) often lack the big advantage of a switch like the CRS504-4XQ-IN: extremely efficient, line rate routing between machines connected to the same subnet (think the 10.10.1.0/24 network I mentioned above) thanks to dedicated hardware. This is really handy for transfers between computers on the same network. If you have the MikroTik CCR2004-1G-12S+2XS and plug your devices into the ports on the front, though, all local traffic has to go through the (slower) CPU. This is known as bridging. If you're willing to accept some performance degredation this with be Fine (@blunden's reckon is 37 Gbps effective throughput and they certainly know more than me), but if you want line rate between devices on the LAN you'll have to get a switch.
(Caviat that I'm very much an interested amateur--this is just my understanding.)
You should be able to use a standard 3M 100G DAC between the server and the gaming PC to accomplish this. For other devices, you can put the server on the LAN as well as the SAN; just make sure you use the 192.168.0.0/16-range IP address to connect to the server from the gaming PC to use the SAN connection and access the higher speeds.
However, you'll still have some speed penalty for local network traffic that goes though the LAN. A router is designed to to actually route traffic between networks. In your case, it would do the work of Network Address Translation (NAT), among other things, allowing you to have multiple IPv4 addresses on your private network and connect those IPv4 addresses to the broader internet. It also lets you move traffic between multiple private networks; say you want to keep your guests on a 10.10.2.0/24 network while all your sensitive equipment is on a 10.10.1.0/24 network but you still want your guests to be able to access a printer on the sensitive network. It's also typically your DHCP server. All those tasks would be the role of the MikroTik CCR2004-1G-12S+2XS in this setup.
However, enterprise routers (like the ones you're looking at) often lack the big advantage of a switch like the CRS504-4XQ-IN: extremely efficient, line rate routing between machines connected to the same subnet (think the 10.10.1.0/24 network I mentioned above) thanks to dedicated hardware. This is really handy for transfers between computers on the same network. If you have the MikroTik CCR2004-1G-12S+2XS and plug your devices into the ports on the front, though, all local traffic has to go through the (slower) CPU. This is known as bridging. If you're willing to accept some performance degredation this with be Fine (@blunden's reckon is 37 Gbps effective throughput and they certainly know more than me), but if you want line rate between devices on the LAN you'll have to get a switch.
(Caviat that I'm very much an interested amateur--this is just my understanding.)
