I am still using the default from 5 years ago, ISC. Anything I should be aware of before upgrading to 26.1.
There are no special considerations I have seen mentioned, use of ISC or Kea or DNSmasq being essentially independent from the upgrade. ISC changes to being a plugin but still transitions as expected, or so I read because I had switched to Kea prior. Use snapshots and upgrade stuff in the order that pleases you. If you are not already using ZFS then consider seriously a reinstallation to employ it, before other work.