I don't have a good answer for you about enabling DNSSEC when using Cloudflare DoT. The sites that do support DNSSEC are few. I saw something the other day that DNSSEC sites are in the single digit percentage of all sites on the internet. I added the DNSSEC detector add-on on Firefox and I can confirm from my own experience that not too many sites I visit support DNSSEC. With DNSSEC disabled on the DNS Resolver, I still pass all of the DNSSEC tests on these sites:
https://rootcanary.org/test.html http://dnssec.vs.uni-due.de/ http://en.conn.internet.nl/connection/ http://0skar.cz/dns/en/This thread does shed some light on the topic.