I see, thank you @pfaffman So, I will try the first step by installing Discourse on a Digital Ocean and see how much it is easy for me to get this step, later I will take care of the other forum.
By the way, Plesk will manage only mydomain.com (the main domain includes Wordpress) and Digital Ocean will have only subdomain.mydomain.com, but I will communicate the both through Wordpress plugin like discourse.org to give the visitors the possibilities to have just one username to use on the website.