With HTMX you can do partial rendering and live updates via server-sent events. This gets you most of the way to the "client side" feel where things load dynamically based on user actions.
The only properly dynamic SPA-like feature in a forum I can think of is a WYSIWYG editor, but that you can build as a web component. Maybe a flexible highlight and quote system would be a bit difficult in pure HTMX (think of the comment functionality in Medium posts). So you'd want to build a few things in client side JS. But the main experience could very well be built with HTMX.