Well, you could do a selective NAT based on the blacklist - or simply have a policy route to next-hop=IP of filter box/sandbox which redirects all destination IPs to self and then replies on whatever service is desired with ye olde sandbox.....
Or you could put a router in front of the sandbox and do dstnat on the sandbox router. So the real WAN router just has a policy that does a route-mark if the source IP of a packet coming in port8 is in the blacklist, then forward to the sandbox router IP as the "default GW" - only route-mark packets coming in port 8 - this way, the standard default GW still works on the replies.
So there is no blacklist currently. All internet addresses need to have this behavior. It will build a blacklist over time. So all traffic needs to be split this way. So I think that disables the selective NAT idea, unless I am misunderstanding it.
Maybe I should clarify one more point, as well.
The computer 1 connected via port 1 will make a request to the internet (say 220.127.116.11:53). The computer on the internet responds trying to connect to the epherical port given by computer1 (like 59234 or something weird). I can see the response
Src: 18.104.22.168:53 Dst:22.214.171.124:59234
being routed to port2Table. Even though the request was started from port1
It is VERY important that 126.96.36.199 and 188.8.131.52 are communicating. There will be one machine on 184.108.40.206, one on 220.127.116.11, etc. if 18.104.22.168 makes a request to 22.214.171.124:80, then the machine at 126.96.36.199 has to answer it. And the response going back needs to be from 188.8.131.52. So when 184.108.40.206 makes a call to 220.127.116.11:80 and 18.104.22.168:80, I can't have masquerade going on causing 22.214.171.124 and 126.96.36.199's IP addresses to be masqueraded (hidden and combined) to like 188.8.131.52 or whatever the ip address on the router's port1 is.
I'm not 100% sure what you mean about the next-hop=ip of filter. There are two different next-hops, depending on which port is being requested.
using the third option, of another router, with dstnat, I think that wouldn't allow all the outgoing requests from port1 to use their real IP addresses, it would all be masqueraded via the additional router, which would be an issue as mentioned above.
I'm not a routing expert. But I love to solve problems. I'll look more into your suggestions and see how the router behaves. It's currently processing over 2k packets / second, so there is a lot of data to look through. But it also means I can see how changes behave pretty quickly.
Thanks for your help. If you felt like offering some specific mikrotik config ideas to help I'd definitely look over them as well.