Personally, after many hours in DOS2 with Larian, there is absolutely NO WAY anyone can say the custom character experience is equivalent to Origin. You got some much more interaction with Origin in DOS2: hidden storylines, extra quests, extra battles, extra dialogue options, extra items even.
There is so much you missed out on by playing a custom character. I played through as custom first before doing Origin and it was almost like a new game with the extra content.
It already appears this is the same in BG3 with the extra dialogue and banter between the Origin and you can tell a lot of the items and triggers on the map are intended for those characters for their stories to be fuller. If you play a custom experience, you don't get that at all just like DOS2.
Larian can't try to say that you will get the exact same experience because you just won't, especially when the Origin characters have unique storylines tied to NPCs and other things that you will meet even later in the game.
If I don't have Wyll in my party, what is my interaction with his patron going to be?