I'm WHMCS admin user/developer already since v2.2 (somewhere in year 2006), did many template customizations, hacks & hooks, provisioning modules and payment gateways (last was The Ethereum payment gateway for WHMCS 7.5.2).
I would love to brain-storm, configure and help you tweak anything on WHMCS.
One of my "Coolest" tweaks was developing a bot that makes it possible to use WHMCS for home management company, this allows Owners get their rental fees paid by their tenenets immediately, in addition to sms notification and reports.
In-case of suspension of the tenant - its locking the door* , or closing water* (using IOT & done by me).
Looking forward to work with you,
Mathew
p.s: je ne parle pas français :(