The “Forward only” option in NIOS-X. You can only tick this if at lease one forwarder is set. If a forwarder is set and “Forward only” is ticked, then recursion will never be used. If the forward 'target' servers go down then you loose the ability to resolve DNS that you are not authoritative for. If you untick “Forward only” then the BloxOne DDI appliance can fall back to recursing directly to the Internet to resolve queries.

NIOS DDNS Updates are processed by the primary member, synced to GM, GM then pushes to secondaries. If GM is offline, secondaries don't get data. If Secondary if offline, GM will queue until it is back online. For NIOS-X, members proxy DDNS update to cloud, cloud processes change, cloud pushes to all NIOS-X members. Cloud only queues for a few seconds. If NIOS-X host is offline, it won't get the updates.

DNS Anycast can work in public cloud but only when using “AWS Cloud WAN” (AWS) or “Azure Virtual WAN” (Azure).

External DNS - 24h TTL minimum

Internal DNS - 8h TTL should be fine especially for static records (should never go below 1 hour). For dynamic DNS records, look at DHCP lease time.