SOCKS proxy over Reverse SSH

I've seen a similar question but it's different enough for the answer not to quite work. I have two Debian Buster hosts - one in a private network (A); one in Azure (B). I'd like Host A to initiate a reverse SSH connection from Host B. Then Host B subsequently to open a SOCKS proxy listener so that connections to that are tunnelled back to Host A. I would like to achieve all of this just by entering commands on Host A. I know that I can set up the reverse SSH tunnel from Host B to Host A (initiated by Host A) like so:

ssh -R b:localhost:a b.b.b.b

a = the listening port on Host A b = the listening port on Host B and b.b.b.b is the IP address of Host B. How can I then get the second part - automatically creating a SOCKS proxy listener on Host B - to work? Is it even possible? Or would I need to create a script on Host B that creates the SOCKS proxy listener whenever netstat shows the reverse SSH tunnel is Established (or similar)? Editted to try and clarify based on first answer.