Какой TLDs я должен использовать для своих записей NS для дублирования? (Требуемая поддержка DNSSEC)

If you are worried about an outage caused by an entire gTLD disappearing, I wouldn't worry about it. The risk is very low and doesn't justify your extra effort.

If you are worried about the a performance different between resolvers having to follow referrals for multiple second-level domains (finding the IP addresses of ns1.example.net and ns2.example.org and ns3.example.info requires first finding the nameservers for example.net. example.org, and example.info which means 3 queries instead of 1), don't. I think that you will measure exactly zero difference here, and besides, most resolvers will cache this information pretty quickly AND it glue records will almost always be given out by gTLD servers as additional records anyway, so resolvers don't even usually have to do these queries.

The only thing you could potentially reasonably worry about is protecting yourself against a mistake that causes your domain example.com to disappear. It could be anything from forgetting to pay to renew that domain to a syntax error in the zone file for that domain that goes unnoticed. Even then things will probably "mostly" still work due to glue records. But to protect against this it's not unreasonable to use nameservers in multiple different domains. But they need not be under different TLDs, really. My advice is: find a good registrar that supports DNSSEC in some chosen gTLD, test it very well, and stick with it!