That error isn't CB Activity specific. Joomla 3 API just returns NULL for object list queries when there's no results. PHP 8 doesn't accept NULL in count() anymore. So that's why you're getting that error. I'm implementing a workaround in core CB for this, but I do not suggest using PHP 8 on Joomla 3 or you're going to constantly run into deprecation issues. There's no need to wait on a fix for this.
As for the Kunena activity you don't need to wait on that either as it's not a migration problem, but a Kunena API issue that's being fixed to have backwards compatibility for Kunena 5 API. Those activity entries will simply just work again in tomorrows release.