I just had my support help me speed up the site by changing the php to 7.3 and for reasons beyond my understanding, it not only sped up the site to a usable pace but also fixed the header warning! That warning is gone now!
So the site is running so well now that I can confidently invite others to test it out.
I wouldn't be able to do this site without your replies. I truly appreciate your excellent customer service!! Not only does it boost my confidence, but it is a bragging point that's been mentioned in multiple board meetings - as we discuss the reliability of the software we're investing in.