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If all of your records are traditional static records, DNS redundancy is pretty simple. DNS includes AXFR for secondary servers to pull the zone data from the primary. You can set up your own primary and use commercial services as secondary, or use one service as primary and another as secondary, etc. If the primary is ddosed, you might not be able to make updates if the secondaries can't connect for axfr, but often that happens on different ips than the public service ips, so there's a chance it still works.

But, it gets a lot trickier if you use any sort of DNS based load balancing or other trickery. Then you've got to set up both services as close to the same as you can and cross your fingers; there's tools out there for that, I think terraform can do it, but there's more focused tools as well. This is a good practice, but it's hard, so it's usually not done.

Top tier DNS services rarely get (succesfully) DDoSed or have other service outages, but it does happen.