Hello!
I'm WHMCS admin user/developer already since v2.2 (somewhere in year 2006), did many template customization, hacks & hooks, provisioning modules and payment gateways (last was The Ethereum & bitcoin payment gateway for WHMCS 7.5.2).
I would love to discuss & brain storm with you on ways we can achieve your requirements.
One way could be using the current ticketing system in combination with products (if you want to charge per task and priority) or integrating with external project management (i have a nice candidate for this option) other option good to consider is using the WMCS Project management module.
Please note, my offer is just a place holder, i will update it with a new offer once we agree on the details
Best,
Mathew