I’m also looking down the line to possibly adding a paid/premium section to my Discourse forum.
A possible solution I’m looking into is using Memberful which isn’t a plugin but a SaaS which integrates their services with Discourse. They have a pretty detailed page on how to integrate the two. You can use Stripe with Memberful as your payment processor which is available in French.
Another way around would be to use a Wordpress plugin such as Paid Memberships Pro which would handle premium member registration (you would need a Wordpress section/landing page on your website), and a bridge/plugin to connect Wordpress and your Discourse forum.
Since your mentioned:
I’m guessing the solution/plugin would also need to include VAT %.
Based on my own experience selling online in Europe, you are going to be leaving some money on the table by not accepting bank transfer payments (some people just don’t have a credit card or prepaid credit card, or know what the heck Paypal is, or are simply uncomfortable paying for anything online unless you are a big brand like Amazon).