Overall I agree that companions don't quite seem aware of each others existance. There is artificiality to the companion content, that's for sure. That said:
4. Examples from other games regarding party dynamic.
- Leliana's song by the fire in DA:O.
- Mass Effect party DLC.
- Garrus and Shepard, beers and shooting stuff from rooftops.
- Geralt/Ciri snowballfight
- Geralt and the Witchers on their drunk detective quest.
Im have to point out that Leliana's Song, Garrus Shepard, Geralt/Ciri/Witchers. First two are content between PC and a singular companion, and Witcher doesn't have companions - those are just story moments. Now those are great character moments - aside from Leliana, which is a moment focused on her character, I think the other are too predetermined to work in BG3. Both ME and Witcher have somewhat or fully predetermined characters. BG3 already for my taste doesn't give my enough urgency as far as narrative is concerned - I wouldn't want to see it swayed even further by deciding nature of relationship between PC and a companion.
3.Vocal approval
Replace the generic text "X approves/Dissapproves" with voiced expressions during the actual quest dialogue. Yes this happens sometimes , but more often it does not. With voiced expressions it will make their feelings more genuine and engaging.
That could go very badly. What you are asking here is replacing a systemic notification with a narrative interaction. Either writes would have to write naturally feeling lines for each such interaction (which even so, having characters constantly say "I liked that!" would be weird considering how often it happens), and using a pool of generic lines could have the opposite effect. And easy example is Pillars of Eternity: Deadfire - while not voiced the companion relationship system, inserts written companion reactions to represent their changes of attitude. How weird those end up feeling ended up a meme among the community, even after the worst offenders were patched out (like a characters showing her approval of giving proper burial to memebers of her religion by clapping and laughing), repetition of lines brings attention to how artificial the system is, rather than making it feel natural.
