I've heard before that one should set TTL for the domains of recovery emails to as high value as possible. The threat model here is that an attacker uses social engineering tactics to gain access to the DNS hosting account to the maliciously change the records to their own servers. In this case, you have longer to remedy the problem
I am also considering a second threat model: if the attacker gains control of the email server and I cannot easily gain back access for whatever reason, I am unable to quickly change the TTL to point to a different server.
Is either of these concerns valid? I know some DNS caches seem to simply ignore TTL, and it is also unreliable since some caches may happen to query for an update just after the hack even if the TTL is set for multiple days.
What should I set my TTL to for the most secure DNS?