Hi,
Thanks for asking. 
About the forum activity:
Here the number of posts for the July 1st to September 1st period (2 months) for the last years when it was still using phpbb:
2011: 2000 posts
2012: 1700
2013: 453 (what happened ?!)
2014: 593
2015: 344
2016: 305
2017: 94 (holy moly)
June 24th 2018: rework of site and forum, migration to Discourse
2018 : 676
(note that we also had 422 posts between April 1st, 2017 and June 1st, 2017 before the migration, so some months are way more active than others)
As you see the activity remains… Very quiet.
The thing is I don’t really act as a community manager, sort of.
I initiated the site and forum rework and I did all the technical stuff, I did some work to (or try to) make more people come. I posted new topics and replies with a few other guys, I advertised our site/forum on various juggling or circus associations sites and facebook page… But that’s not my thing and I won’t do that on a long term.
My co-worker (which did the previous website and was the admin of the previous forum) work with a government approved unicycling sport federation and I think their job is promote this sport, so it should be their job to do such a thing because the website and forum are officially affiliated with this federation.
I don’t know how active they are and I don’t want to interfere with what they do. As the webmaster and nothing else, I think my work is done and I’ll let them do what has to be done if they want this site to be more popular.
French unicycle Facebook groups are mainly “local” groups such as associations or friend groups. Since they often talk about local events, they have no need to post elsewhere.
Plus, there is still this issue that unicycling isn’t as popular as it was at the beginning of the 2000s and we can’t directly do something about that!
I’m sure we could improve the forum activity in some ways, but I’m no expert and I don’t want to spend more energy on this, I’ve already done tons of work.
Finally, when we search for “monocycle” (French word for unicycle) on Google, our website comes only on the 6th position, which is a real shame as it has been the only non-commercial unicycle specialized website for 20 years and has tons of content.
With the site rework, I’ve been improving some SEO related stuff on the new website and we have to rework some pages.
For example, even if it’s the only unicycle resources French website, it seems that it never had pages which describe what is a unicycle, how to learn, what to buy or basic stuff like that! 
So, stuff has to be done, because a more popular website will make a more popular forum.
That said, moving to Discourse was a necessity and I’m very happy about it. Users (and admin!) like it.
And specifically about the Wordpress/Discourse combination: I’m not sure how I can see the real benefits but there is no downsides. It’s cool that the site news are also posted on the forum so users can reply from the forum and we see their replies on the main site. Also I show the last forum topics on the website too with a custom plugin, but I don’t know how all of this improves the site popularity.
So I’m not sure how I can reply to your question !