Then only use that password manager for PhpBB forum...
You care way too much for an account that you currently don't even have... is is really necessary? Worse case, you lose it and you are back a your current state, doesn't seems so bad to me.
> currently don't even have
And won't have with the current login method.
I'm not trying to convince you to follow my path here, I've just been trying to explain why I do what I do, in an effort that the guy advertising a forum I'm interested in might improve things enough to let me (and those like me) in. I understand that there are ways to do this, and that my way is an active choice.
If it comes to it, I'd rather go without.
If an identity provider has issues, the window of opportunity ends with the rollout of a fix.
Not to mention that I trust the security of my identity provider a lot more thorough than Mr Bobbins phpBB instance. Audits automatically instigated by changes in country, ISP, device, browser. Logs kept. Dynamic secondary and tertiary factors.
It's toxic and unnecessary data. Let somebody else handle it.