sp-request | Simplified SharePoint HTTP client | Content Management System library

You could do

Turns out the incorrect part is the MediaType, which has to be binary for the server to correctly read the data. Code corrected:

SSL server (if configured correctly) will send certificate chain (except root CA certificate). You can verify it here.

Openssl did not fetch these certificate but it got them served when initiating ssl connection. You can read more about s_client behavior in openssl documentation

I don't know if it performs OCSP verification but I doubt it. IMHO (based on The s_client utility is a test tool and is designed to continue the handshake after any certificate verification errors.) it does not perform any validation by default at all but you can at least enable CRL checking by specifying argument -crl_check_all

You could use GetElementsByTagName as suggested by WOxxOm's comments. It's elegant. But potential issues could be caused by changing xml structure, re-ordering or multiple connectionStrings.

You could dot-index into the XML. The advantage is that it is more explicit. The disadvantage is that it is more explicit.

If you change the path to the connectionString this will need to be updated where as GetElementsByTagName will not. However this is better for explicitly defining the tag you are interesting in and to give you convenient access to the other pieces of information within that tag. I find it is generally more intuitive.