As mentioned about 50% of our unindexed pages are P= post objects and a high percentage of the remaining 50% are low-grade topics,
lacking rich content, i.e. media, media descriptions, or posts with dead/broken image links. So yeah, GSC wasn't interested.
Our Sitemap only lists T= topic objects. Strangely, if we check if the T= object is indexed, we've found that about 90% of the ones Google says aren't indexed, show as Indexed within GSC; the reverse of what their results show. I don't know if there's a delay in their system or if something is broken in GSC, but we've given up on trying to place too much emphasis on Google's Search Console results. Some stuff is helpful, but most of it is just informative or blatantly inaccurate. Running 'Validate Fix' for items it says need fixing that aren't broken, doesn't do any good, because the validation will often fail; the pages aren't broken in such a way that causes them not to be indexed, so there isn't a fix in Google's eyes that will suddenly result in more pages being indexed.
Just look for trends or spot check things to determine if you can do something better on your forum or if it's out of the scope of what can be changed to make GSC happy. Sometimes GSC is best utilized for those types of functions.
As sophisticated as Google thinks they are, I get the impression that their high paid 13 y.o. A+ brainiacs or 50 y.o. programmers with the mentality of a 13.y.o. don't think like you and I, so good luck with the salad of information and 'fix' options that tell you little and that give you the impression will genuinely offer greater indexing of your PHPBB site. I keep hoping that one of their How-to videos will apply to something I can do on our forum, but 99% of the verbal diarrhea that comes out of their videos is impossible to make any sense of. Worse yet, they often don't date their How-tos, so you'll think you've found a new trick, but it often turns out to be an outdated article that no longer applies.
I've contacted them repeatedly showing the inaccuracies, but nothing is ever done about it. Draw your own conclusions as to how important your feedback is to them from that.
