They support cPanel, and that is it.
cPanel has pretty much everything in it that the majority of website owners ever need.
They're not going to fix your server for you (unless cPanel is causing the problem)
There are two things when using cPanel, the hardware (looked after by the host, Linode, DO, etc) and the software, all of which once you install the Centos on your Linode / Droplet, is all then managed by WHM. I have used cPanel support to fix OS updates via the WHM console, and they absolutely fix such issues because they provide the feature. If it breaks, they fix it. I've had them do it for me.
they aren't going to do updates other than the ones that cPanel handles, they aren't going to apply security patches that aren't cPanel related
Updates for both the OS and cPanel are done via WHM. Any issues in either process and cPanel absolutely fix the issue, update the system and go forward. I had an OS update cause DB issues, taking out a site, they fixed that too.
they aren't going to do the myriad of other things that a server management company would do.
Like? When the hardware is looked after by the host, the software by cPanel, what else does the average website owner need? Anything custom a host charges you for nowadays. Often even when you have managed hosting. No different if using cPanel... you get someone to do any custom aspects at setup, or once a year if needed, just like you outlined above.
Otherwise, hosting management is not needed.
There are no responses within minutes.
Most of my responses have been within minutes from cPanel support. They're usually very quick when you outline the issue, give them direct access via their system, and away they go to correct the issue. They usually tell you when starting, then again when fixed. There is no need for discussion with them and their instant access system.
You're a host. You have one view. I have another. Managed hosting is not a requirement nowadays like it used to be. Its cheaper to buy SSD VPS with a cPanel license direct, once setup, anyone used to cPanel can then manage their day to day aspects. Setup is always the tedious part... beyond that, there is no need to touch a server more often than not. cPanel even make setup super easy... one line of code from the built-in SSH client.
There are lots of cases where managed hosting is ideal, custom work may be needed by larger sites... but for the majority, install Centos, install cPanel, adjust the default settings to suit, upload site and away you go. If already using cPanel... import it and everything just keeps working.
I like my custom NGINX setup... but I do a similar setup for people who like the ease of cPanel... once done, it just keeps working for them.