@SeanKim ...
I, too, do not know much about phpBB forums (I think I may have tried it a long time ago when I was managing my church's website). Are there tools that a Canvas discussion topic is missing that phpBB has? If you were doing any graded discussions, it might be a bit harder to do (IMHO) because of having a separate discussion board interface in your course when it comes to grading.
Have you checked out any of the LTI external tool integrations that are compatible with Canvas? I don't know much about these (and there may be a cost associated with each), but I searched the EduAppCenter website, and I found the following that might be worth investigating:
There may be others available on: Edu Apps (eduappcenter.com).
To address the question of HTML tags allowed in the RCE (Rich Content Editor) of Canvas content pages, there is this document:
Canvas HTML Editor Allowlist
If you are using HTML tags that aren't on this list, then they will be deleted...as you've noticed.
I hope some or all of this information will be helpful to you. Sing out if you have any questions...thanks!