Ok @jota I guess it’s the best move as nothing evolved for 2 days 
And indeed, I have plenty of questions referring to your guide:
a/ About “Getting Started” docs.bitnami.com/google/apps/discourse/get-started/
Do I need to need to connect to phpPgAdmin, postgresql and redis as I don’t want to access that level?
b/ Do I need to install plugins before everything else, as in “Configuration” docs.bitnami.com/google/apps/discourse/configuration/ ?
c/ Same page, do I need to do all the Apache, php, phppgadmin, postgresql and redis configuration? I don’t know all those modules and stuff, which are mandatory?
d/ Same question for the php, phppgadmin, postgresql and redis pages of “Administration” docs.bitnami.com/google/apps/discourse/administration/ , which are mandatory?
e1/ In “Update The IP Address Or Hostname” docs.bitnami.com/google/apps/discourse/administration/update-ip-hostname/ ,do I need to set it both with the bnconfig tool and the hosts file?
e2/ If I use the bnconfig tool, is it ok to enter
sudo /opt/bitnami/apps/discourse/bnconfig --machine_hostname www .ludgef.com
without http or https? (added a space because I cannot post links)
f1/ I saw my current Discourse version is v2.2.2-dirty. don’t you use stable version for production?
f2/ I also saw there a new version 2.3.0.beta4? (still not stable) Shall I update before anything else or after?
g1/ Do I need to do both procedures in “Enable SSL In Discourse” docs.bitnami.com/google/apps/discourse/administration/enable-ssl/ AND in “Enable HTTPS Support With Apache” docs.bitnami.com/google/apps/discourse/administration/enable-https-ssl-apache/ pages? In what order?
g2/ In “Enable SSL In Discourse” docs.bitnami.com/google/apps/discourse/administration/enable-ssl/ , do I need to “Update the HTTPS port entry in the database” as I use default?
g3/ In the same page “Enable SSL In Discourse”, I don’t understand the phrase “Then, force HTTPS for all Discourse links using one of the following options:” as the following looks more like steps and not options.
h/ Do you think I can remove my current MV in GCP, redo an install and be able to link the new VM to my current fixed external IP, or should I close and keep the old VM until the new one is up and running?
Sorry for this mountain of questions, but I like when I understand 
My wish list for this forum is : https only, SN logins, analytics (incl), PWA (incl), Spam & Ads plugins, PageSpeed (opt).
I will proceed in a few hours, leaving some time to answer and I need to do other work now.
I will note my doings as I go.
Thanks again for your precious help