Usman Asar

Yes, BigBlueButton (BBB) that moodle is using mostly to do live interactive sessions and record them afterwards, but BBB doesn't runs on normal HTTP/PHP capable server, it runs on Apache TomCat, so even if your setup is Linux based, you still will be setting up an additional server just for BBB.

I recommended YouTube/Vimeo, to upload the recorded session, between the two, YouTube does offers Live and can be recorded to be served later on, Vimeo however wont allow live stream on basic package.
Both YouTube and Vimeo have limitations in terms of interaction of users, that you have to keep in mind, however both were recommended based on unlimited size upload to be watched later, as on a cloud server, you should keep in mind space and bandwidth will end up costly. Therefore Improvise

session timing isnt dependent upon memory, but number of users are.
if you plan to use BBB as conferencing and recording sessions, then you'll be setting up additional server based on Apache Tom Cat, other than your standard HTTP serving web server.
you can, however, use third party video site (YouTube, Vimeo) to share recorded sessions.

Helen, upgrade process you followed was absolutely right to upgrade it perfectly, despite a huge gap in between. I haven't personally read through the upgrade documentation (or may be I had a decade back on my first upgrade), but I suspect a crucial thing missing from documentation is looking into plug-ins first and that is what I recommend people upgrading, to make a list of additional plug-ins installed and look into plug-ins directory to see if the plug-ins are updated to the moodle version they are updating, in most cases plug-in developers do upgrade plug-ins before hand or right on release of new moodle version. however moodle 3.9 is exception for still many plug-ins are getting there to be supported by 3.9. This brings to Adaptable which still isnt, so people switched to Moove theme (as of now, most downloaded theme) so assumption is people switching to that.

This, I recommend to you as well, to switch theme as of now, you can choose to see which theme suits you well by looking into most downloaded plug-ins (https://moodle.org/plugins/stats.php) and you'll find some 5-6 themes listed there that were most downloaded. which ever suits you well and supports moodle 3.9 as well.

@Rick, Adaptable is a fork of BCU (Birmingham City University) theme, later developed to be used in Coventry University where Jez (Lead Developer) moved to from BCU, that is why I see Helen's affiliation to apply it on Coventry City Council