You can exclude the devices using the C-7. Put C-7 in exclusion mode, do whatever you have to on the device to cause it to exclude (normal exclusion process for device). C-7 will report unknown device excluded.
If you have a backup of your c5 you can restore it on a new c7. Then follow the instructions in the post from Hubitat. A search for migration should bring it up. That way you can at least save your rules.
A multi-hub configuration depending upon the size of your house can be useful. Lower overhead, greater stability if you keep most of the rules local to each hub.
I used to have 2 C-4s - one on the main floor covering basement and main floor and the other the upstairs. Worked really well and I recommend it. Now I have a C-7 all my Z-wave devices are on it and my Zigbee devices are on a C-5 - that appears to be working okay as well but we'll see. Am repurposing one of the old C-4s as a cloud/network device hub no ZW or ZB.
For a "secondary" hub you'd need an ethernet run (or alternative like powerline or moca) to where ever it would be located.