Bugs, balancing issues, and other relevant feedback can and should be shared with Creative Assembly. Once the game mode launches, the developers are asking that players (respectfully) give feedback. Instead, Immortal Empires will be available at launch for everyone. You won't have to opt-in or sign up for something to play it. Total War: Warhammer 3 Immortal Empires will feature the beta tag until the developers feel it no longer applies, but this won't affect how it's accessed.
Total War: Warhammer and Total War: Warhammer 2 both had extensive post-launch support, and Total War: Warhammer 3 shouldn't be any different. These will take time, and when combined with the extensive post-launch DLC outlined in the roadmap for Total War: Warhammer 3. So Total war, loving all that yall do, but will there ever be a chance yall could set up a content team to continue adding factions, lords, or uniqueness. In particular, the developers have highlighted issues with how Corruption works, lighting quality on maps from the first two games, and balancing factions across the three games to ensure there's a generally consistent playing field. Creative Assembly says that Immortal Empires will be "content complete" at launch, but that there are countless small things that will take time to fix. The reason for this is that the developers want to provide Immortal Empires to fans as quickly as possible, but are also blunt about just how complicated this undertaking is. Immortal Empires is launching in an open beta format.