BG1 and 2 have achieved the feat of being a very faithful transposition of AD&D into video games.
I just have one question for those who have played Early Access before: By playing BG3, will I feel like I'm really participating in a D&D5 campaign, with the same rules and the same possibilities?
Thank you in advance for your answers
BG1&2 wasn't exactly what I'd call "very faithful" to AD&D, but relative to BG3/D&D5e, they certainly were. If you played DOS2, more than playing a faithful D&D rendition, with BG3 EA you are likely to feel like you are playing DOS2/3 with a good D&D5e themed mod. Don't expect faithful implementations as Larian's paramount concern is making a fun game. The subjective interpretation of fun is heavily skewed in favor of DOS-style of silly fun though...with not so much regard for immersion breaking or balance breaking aspects.
Embrace the cheese, or you will have a hard time of this.