Yes, the bridge 5.2.1 works with phpBB 3.3.12
You can upgrade phpBB as usual.

FYI: the light bridge version works with Gantry now. Usual p8pbb bridge doesn't and currently, there is no way to make it compatible.

Sorry, I still don't understand you. Please learn how a web server and Joomla works first, you need at least basic developer knowledge.

Earlier I asked can I get any website link that they have used "MightySites". For get an idea how it works?

... over to Joomla, set up EasySocial with the phpBB bridge and make a copy of my current phpBB and set it up on the sub-domain while my live site works away as normal. The question I have is this, If I make a copy of my current phpBB and set it up on the sub-domain can I later merge all the new ...

... re-map all synced fields in bridge settings after upgrade. Don't forget to update CB plugin and EasySocial application as well. And... Yes! It works with Joomla 4 Alpha 5! Note for websites with Community Builder: first disable p8pbb CB plugin, next update bridge and install new CB plugin, ...

So that means this works? Is it also working well with integration with EasySocial?

Actually, I've just tested newest Gantry5 and Helium template, it works fine and the old bad code is improved.

So, Gantry5 should not be a problem anymore.