I'm setting up a small, relatively isolated network attached to my home network, for me to try to learn more about setting up routers, clustering, etc. I'm running into an issue though.

It's a few Raspberry Pi boards connected to a MikroTik RouterBOARD 750G r2.

I've been able to successfully set up port forwarding to be able to ssh into one of the Pi's, but now I would like to try to access a camera stream from another. I've tried to set up port forwarding to the second Pi, and the graphs on the Firewall NAT rule seem to show traffic (I can see a spike whenever I try to load the page from my desktop), but there is never a response.

To troubleshoot, I connected the Raspberry Pi directly to the main network (bypassing the RouterBoard) and could connect just fine, so I know there's something with my router rules. I've spent several hours working on this and just can't find a solution.

I've attached my configuration.

Thanks!

Not enough info. Posting config is good, but we know nothing about your network, we need to know about all involved devices, their IP addresses, what exactly should connect where, etc. It's obvious to you, but not to random stranger.

It's a pretty minimal setup.

Home Router
|---> Desktop PC (192.168.68.104)
|---> MikroTik Router (192.168.68.137)
|---> Raspberry Pi #1 (192.168.88.100)
|---> Raspberry Pi #2 (192.168.88.101)
|---> Camera serving on port 8090

I can SSH into RPI #1 at 192.168.68.137:22, and directing the browser of my PC to 192.168.68.137:8090 shows packets being sent and the hit counter on the dst-nat rule increasing, but the request times out because there's no response.

Bypassing the router looks like this:
|---> Desktop PC (192.168.68.104)
|---> Raspberry Pi #2 (192.168.88.134)
|---> Camera serving on port 8090

Going to 192.168.68.134:8090 shows the camera video stream.

It must be something on RPI. Your dstnat rules are all same, only with different ports, and there's nothing else in your config that would treat different ports or addresses differently. Check RPI's network config, make sure it really gets it from DHCP. Ping router (192.168.88.1) or something in the other network. Check RPI's firewall if it allows access from elsewhere, of if there's perhaps some ACL in camera software.