You can set NS records for subdomains though... I have done this for a couple of domains where i have internal.<domainname> and that points at Route53. I also use Route53 for my Reverse IP lookups, since Cloudflare does not seem to support this (unless i missed something).
On an related note, what does Route53 give you that you cant do on Cloudflare?
Ahh ok. That's not so bad. It's really only for one 'learning' domain or such where I set it elsewhere. But it's by far the minority of the time that I didn't just use Google's built in one (or used GCP's) for the API which Cloudflare has
On an related note, what does Route53 give you that you cant do on Cloudflare?