Lemmy is essentially also a federated forum (Really a link aggregator). The only real difference from a link aggregator to a forum is that forum comments are in order by date and time while a link aggregator is in a tree with the replies to other comments.
Essentially add a button next to chat and a setting in the account settings to sort the comments the exact same way as in "Chat mode" except it will show the oldest comment first up top and the newest comment last at the bottom like a forum thread.
(Describe your proposed solution here.)
Adding a toggle button on every single post called "Forum" or something like it to sort the comments by date and from oldest to newest. And for comments that are commenting on other comments to have a link to the parent comment and the parent comment in quotations in the comment child comment.
And to also have a setting in the account to have all threads sort comments this way if the user chooses without needed to press the "Forum" button every time.