Thank you for this feedback! Thanks to these suggestions I've got the connection working now. Here was the process:
I'm using DAC cables (Cisco SFP-H10GB-CU1M). First I tried swapping one out for another and the problem remained. (The cables had already been working fine between the MikroTik and a Dell Optiplex that I have here, so I knew the MikroTik recognized the cables.)
Next I went into the MikroTik admin settings. The connection with the Qotom Mini PC running Proxmox was recognized by the switch. I looked at all the menus within SwOS (SwitchOS), comparing the Qotom connection to the Optiplex connection, and all of them looked the same. The only difference was on the Stats page, which showed regular Rx/Tx activity for the Optiplex. The Qotom, however, had no Rx activity, and Tx activity that hovered between 0 and about 10k.
Then I began swapping the cables into different ports on the Mikrotik switch, updating the Linux Bridge port in Proxmox each time. Finally I was able to get a carrier connection with the cable inserted into the fourth SFP+ port, which was nic8 in Proxmox.
After I saw that was working, I tried swapping the cables on the Mikrotik into different ports to see if I could get it working in those configurations. I wanted to rule out the possibility that the switch wasn't responding correctly with two of these DAC cables in adjacent ports. Unfortunately I couldn't get any of those working, but once I moved the Proxmox connection back to nic8, the connection was recognized.
I'll report back if/when I determine *why* only nic8 is working, but in the meantime, success!
