In DoS2 none origin hero’s lost about 1-2 hours of content per act, so over 6 origins tav would be shorted somewhere between 6-12 hours of content if BG3 handles it same way. Personally I think that is a lot of missed content as it is 15-28ish% of the 75 hour playtime. Start origin is my plan
By that example the other origins would miss out too. The Red Prince would miss out because he didn't get Fane's content or the others'. So Gale, for example, is missing content too. So then Gale would be missing 4-10 hours?
each origin got 6-12 hours of content related to said origin story mostly though unique quests. So, as long as you picked origin, you got those 6 hours, but if you pick custom, those 6 hours won't exist since custom didn't have story quests just for him.
But we can access that quest via that origin character being our companion, so... how much content do we miss then?
Playing as Lohse and recruiting Red prince, Sebille and Fane - you played through Lohse story and had experience in helping you companion do their quests three times
Playing as custom, you got no quests for your own, and you got to help your companions three times.
Sure, you could play custom, Red Prince, Sebille and Fane, then custom, Beast, Ifan and Lohse, but that would mean replaying whole story just to see other 6 hours you missed and you'll not experience any companion story from your own POV.
Not everyone wants to play everything. Some of us want to play most effiecient way of experiencing the story. I have over 500h and I felt short of experience when I end up picking custom characters. I enjoyed DOS2 and would replay DOS2 for 500 hours, but custom origin definitely felt short.