Thanks for your detailed answer.
After reading this and some other articles I think I will go for the sub domain. It will give me the advantage of placing the billing / support portion of the hosting on a different server, which I see as more secure and also beneficial if customers require support if the hosting server has problems.
What I didn’t realise is that I could use the same dedicated IP for certificates on the sub domain and main domain (if I understand correctly).
With regards the cPanel, I don’t want my clients to see the site5 server address when connected securely and be able to use my domain name. Can this be done? I thought I may be able to use a cert on say client.domain.com and use client.domain.com:2083