old WHMCS update process planning

[Walther]

Problem:
I have a fairly old WHMCS installation (WHMCS 7.10.2 and PHP 7.3.3), which is now at the end of its useful life.
The current WHMCS installation is used for both the website (service presentation pages, price lists, “about us,” etc.) and the customer area.
(It seemed like a good idea at the time, but it’s not: it’s a mess, management is rigid, there’s no SEO optimisation, creating multilingual pages is a mess…)
So I was thinking of installing the new WHMCS in a different directory (e.g., my.example.com) and developing the main site with WP on example.com.
This way I could:

• leave the old WHMCS site running on example.com for as long as necessary
• develop the new customer area on my.example.com
• develop the new site offline
  Once everything is ready, I “just” need to:
• delete the site on example.com
• install the new WP site on example.com
• migrate the database from the old WHMCS installation to the new one
• set up a series of redirects in .htaccess from the old URLs to the new ones

My questions are:

• Did I miss something along the way? Is there something I haven’t thought of?
• I'll need to migrate the database from a WHMCS 7.3.3 installation to 8.13… which tables do I need to migrate? Can I do it from phpmyadmin, or are there differences in the database structure?