Stuart Newton,
Firstly know that expansion is great but costly, if you have running capital to inject into your expansion go for it!
However, if you only have a handful of clients (30 customers), you may bite off more than you can chew as it requires upfront capital!
Basically yes, standard reseller hosting plans do not have the ability to create sub-resellers as its already second tier hosting. (At least none of the traditional panels that i know of!)
Tier 1:
Hosting Provider > Clients
Tier 2:
Hosting Provider > Reseller > Client
I dont think that a multi-reseller hosting plan exists on any of the popular control panels such as cPanel, DirectAdmin etc.
If you want to offer your very own reseller plans, you would want to obtain a VPS/dedicated server(s) and load it with your desired control panel. Setup your resellers plans and you good to go!
Herewith a few resources that will help you setup:
https://docs.cpanel.net/installation-guide/install/
https://docs.cpanel.net/knowledge-base/accounts/guide-to-reseller-accounts/
To sell cPanel/WHM licenses, you would need to register for their partner program. (Note there is a minimum charge of $250 per month)
https://cpanel.net/partners/
To sell WHMCS licenses, you would need to register for their reseller program. (Note there is a minimum charge of $68 per month for 5 Starter licenses regardless of usage)
https://www.whmcs.com/resellers/
Hope the above information helps 😀