nslookup, dig, and host only return half of my TXT records

It is not possible to do what you're asking for for a domain you do not own.

By design and due to privacy considerations, you can't list the TXT records of all subdomains unless you have special permission within the DNS server to do a zone transfer (AXFR) lookup. Unless you have this zone transfer permission, all records that are not under the @ (apex) domain need to be queried by each individual subdomain they reside on.

This prevents people from discovering subdomains and DNS records that are not intended to be public.

If you do control the domain, your DNS provider should have instructions for allowing zone transfers to a particular IP address. If you enter your IP address there, then you should be able to use dig's axfr mode to list all the DNS records - but by the point you log into your DNS provider, you may as well just view the records from their web interface, unless you are trying to automate something.