With a compatible network card, you can turn an old PC or thin client into a router or even run it as a virtual machine.

One of the biggest benefits of pfSense is that it can be installed on various types of hardware. Also your configuration is questionable in overall, but above solution should work. pfSense is a free and open-source firewall and router. Add DST-NAT rule in Mikrotik so when reaching 172.18.0.1 your src-ip is rewritten to 172.18.0.3. This means you need to edit existing firewall rules. So in Proxmox packets are assembled into 64k buffers which seems to create MTU issues for the CHR. PfSense: Software Stateful Firewall Similar to RouterOS firewall, a stateful firewall open source pfSense software filters the traffic. Allow access to 172.18.0.1 in Mikrotik firewall from your LAN.

Hyper-V has completely different buffer assiggnment for the way it moves packets through the hypervisor and so it actually does not suffer that issue and you can put jumbo frames into Hyper-V without issue. Hyper-V on the other hand does not do it that way.

'There is a challenge there and it creates a serious throughput/performance issue when you're using this in ESXi and KVM Proxmox because of the way the buffer are arranged transfer the packets out. Let me cite explanation of the problem here from a talk given by Mikrotik's team member:

Pfsense on mikrotik