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The DNS TTL only affects devices that have queried the name. If a device hasn't requested an IP for the host name it will not have the address in it's cache. So, let the new device boot up, get it's MAC, create the static entry and point the host name to it. Then, when some other device queries that name, it will get the static address.
I think you may be imaging problems that don't exist. You'd only have to worry about DNS TTL if a host name had been in use and then the address changed. The TTL only affect pfSense for host names learned from elsewhere. When you control the host names on a local DNS, for local devices, there's no cache in pfSense to worry about, as it will rely on what's saved in /etc/hosts.