Do I have to rent a modem/router combo from Sonic or may I use my own and save the rental fee? Seems like the modem/router pay for themselves after a year or less
Yes, currently you can opt out and return the modem once your service is up and running, after it's completed it's automatic optimization process, a week or two.
But you'll lose the benefits of support visibility of the CPE, plus the included Wire Plan, which covers the cost of any necessary truck rolls or inside wire repair. And of course the end-to-end service assurance and next-day replacement shipping in case of equipment trouble.
Obviously I'm hopeful you will retain the bundled equipment, as I believe it makes the Fusion service better and easier in many ways, but today it is possible to opt out and return it to avoid the $6.50 expense.