1) Include a link to dnsviz.net to check on the DNSSEC status of domains. They've already done all the work and it would be a nice integration.
2) Something that I wish more DNS operators understood is the concept of shared fate between authoritative name servers. Shared fate can come in the form of same AS, same upstream, same parent domain, etc. Operators might think they have redundancy when in fact all their servers are located in the same AS, for example. If there is any way you can highlight this or show this it would be useful.
3) I didn't try looking up a phishing domain, but displaying whether a domain exists on popular block lists would be awesome.
I love your attempt at understanding all the TXT RRs that have spread across the DNS in the last 10 years. What a mess.
You're right in that this is a rabbit hole. You could spend the rest of your life building this and never actually completing it, be careful!