Earlier this year,
the Internet lawyer Arnoud Engelfriet wrote a
blog post about exactly this topic. As it is written in Dutch, I will summarize it here:
As you also said, deleting posts breaks the flow of the archived conversation and it makes your archive incomplete. This is a problem for the freedom of expression and information. But
Art. 17(3) GDPR includes an exception to the right of erasure for this situation. So posts do not need to be deleted.
However, profiles are not included in this exception. So they must be removed, but they can be pseudonymized. For example replace the username with user89432, and remove all details from the profile.
If other posts contain the nick of the author of an anonymized post, that is considered an journalistic, academic artistic or literary expression, so
Art. 85 GDPR would apply, so the right of erasure does not apply to that.
Bottom line: you only have to pseudonymize the account, if that person wants to be removed from the forum.