The Roon installer on a Mac (or a Windows machine for that matter) installs two programs. Roon desktop client (just called Roon) and Roon Server. Since you use your Synology DiskStation as your Roon Server, you should not ever need to run Roon Server on the Mac.
Having installed Roon on the new Mac, the only ‘setup’ required is to run the Roon client and connect it to the Roon Server already running on the Diskstation. Everything else should have stayed the same. It certainly should not affect ARC connectivity (port forwarding). Re-runing setup will not do anything to fix the issue.
I’m guessing that that rule is left over from a previous uPnP configuration request. Has the ip address of your Synology diskstation changed recently? Such things can occaisionally happen when ip address allocation is left to DHCP.
The ARC error diagnostic that you posted mentions ‘ConflictInMappingEntry’. This is the cause of the issue. It is saying that it cannot honour the uPnP request to forward traffic because traffic for the specified port is already being forwarded to a different ip address. I’m assuming your Roon Server is configured to use port 55005 for ARC connectivity (in the Roon->Settings->ARC settings page as in the image I posted above). This would explain the conflict with the rule shown in your router screen shot.
I would start by rebooting your router - then wait for it to stabilise. Hopefully, this will clear out the previous port forwarding rules created by uPnP. The uPnP Portmap Table should, at this time, either be empty or it should show a new forwarding rule for the Roon Server with the correct ip address.
If Roon is still reporting a conflict in mapping entry, then restart your Roon Server. I believe that you can do this by stopping and then restarting the Roon Server service on the Synology - you should not have to do a full Synology reboot (although that would achieve the same result).
If you still have the same problem, and the 192.168.0.16 port 55005 forwarding rule has not gone away, then the easiest solution would be to change the port in Roon to avoid using 55005. Try using something like 60000.
As a matter of interest, could you post a screen shot of your Roon->Settings->General page showing the details like (Note: you should block out your email address as I have done before posting):
