Without any customization, viewing flagged mails as conversations is tricky and requires multiple clicks.
One easy way to get it in 1 click is to
- first create a custom view to show only flagged mails…see: https://discventionsassorted.wordpress.com/2019/12/05/create-custom-views-in-outlook/
- Next, in that custom view, group by Conversation (view settings)
Just use arrow keys to navigate flagged items and right arrow to expand conversations and “insert” to mark items complete.