In the further spirit of asking good questions - specifically, in continuing to work on the problem, I finally found a relevant Netlify support post: [Support Guide] Can I host my site on Netlify but keep my DNS at my domain registrar?
The key for me was that
a) I needed to create both an A record and a CNAME record in my Custom DNS settings at my domain host (Pair Networks)
b) The A record needed to specify @ as the subdomain name, and point to the IPv4 address of the Netlify load balancer (currently as of Feb 2023, that is 75.2.60.5). Using @ as the subdomain was the key point I had been missing
c) The CNAME record needed to specify www as the subdomain name, and point to apex-loadbalancer.netlify.com
Then, after waiting a bit and refreshing the Domain Settings page for my Netlify app, the Check DNS configuration warning has disappeared from the apex domain name.
I don’t yet have the SSL/TLS certificate, but now I believe that is a matter of waiting for DNS propagation, rather than a failure of the DNS configuration in the first place.
And both psychdevsolutions.com and www.psychdevsolutions.com point to the website deployed in my Netlify app. Yay!
And my domain remains hosted at Pair Networks, which I wanted all along (I’ve been a customer for literally 25 years).
Anyone, please feel free to comment/suggest ways to improve the process, etc.
–David