your character's 'background' isn't their culture though, it's closer to a profession, you keep trying to put culture into it but that's not what purpose it serves.
The Background is the characters culture. The playtest Background is an enormous amount of design space. It can represent broad and specific experience within a culture. Different cultures make different experiences and professions prominent.
A character that grows up in an environment will learn from the experiences there. One speaks a prominent language from the culture there. In that culture, depending on the culture, one might grow to be a farmer, a student, a criminal, hunter, a merchant of some kind, and so on. All experiences are in the context of the values of that specific culture and the opportunities there.
For example, consider the Uda Drow culture of Menzoberranzan. Any experience of that culture is a Background. One might grow up as a successful student of warlockry in its magical acadamy for males, skilled in its demonological Religion and fey Arcana, and signing the Drow silent language. That Drow Warlock academic culture values Charisma (score +2) and stealthy Dexterity (score +1). There is even a feat to represent a specific arcane accomplishment, or family upbringing, or other sector of Menzoberranzan culture − all understood in the context of Uda cultural values.
An other Drow might grow up in the military academy to make and use Sleep Poison.
An other Drow might grow up in the Lolth seminary for females, an acolyte with priestly capabilities.
An other Drow might grow up as an Underdark Farmer.
And so on. Both the DM and the player can detail what Menzoberranzan culture is all about.
A character who is a Human/Elf in Menzoberranzan will have a different cultural upbringing than one in Neverwinter.
my example was meant to explicitly give the trait to half-elves, it isn't something either of the original parent species were meant to commonly achieve but represents the half-elves being something unique between them.
But rather than assign the cultural traits to a species (which is kinda racist in principle) − or in this case to a multispecies biology − those "half elf" traits are legitimate Background experiences for that individual as well as other Human/Elves who grow up in a similar cultural circumstance.
i think that expertise needs to be much more common, it's only considered so powerful because it's so rare and the glass ceiling of bounded accuracy meaning for everyone else there's only a restricted set of outcomes for rolls, but if it were more common it would create a set of target DCs for actual experts.
The Background design space includes a level-0 feat and two skill proficiencies. Spending both that feat and one skill is enough to gain proficiency with one skill that one is already proficient in. Spending both skill proficiencies along with the feat, is enough to gain a new skill proficiency and expertise for it.
I also want to see tool expertise be common, especially within various Human cultures each making certain tools prominent. For example, a Human-majority seafaring culture will make expertise with ships common, as well as tend toward cosmopolitan and multispecies. To gain a tool expertise, use the Background tool proficiency to gain the tool, then use the feat and one skill to gain the expertise for it. That character grew up focusing and mastering that tool and all the activities associating with that tool.
To be clear, I feel it is ok to say the fantasy Human species has a biological instinct to learn new information. This instinct expresses itself as Human curiosity, a need to discover, understand, and find safety in the context of some unfamiliar phenomenon, and as Human competitiveness, a need to demonstrate mastery in the phenomenon compared to other Humans. This learning is open-ended and can involve anything, any skill or tool or weapon. Proficiencies and expertise are notable Human impulses.
At the same time, Human-majority cultures will value proficiency and expertise, and even Non-Humans who grow up in such culture will tend to acquire these Human values, and take on proficiencies and expertises alongside Humans in the context of Human curiosity and competitiveness.
you can't just explain everything away by saying 'if it's in their culture', half-elves don't come from species that have any culture to deal with outliving generations humans don't have to deal with it and neither do elves, but half-elves do and they don't get guidance on how to deal with it, they are both and they are neither, they are included but they are other, this disconnect is exactly why they form their own communities because staying with either humans or elves inherently others them due to factors of their fundamental biology, and even if they do live in communites of other half-elves they can't deny that they ultimately did decend from these two other species who have totally different experiences.
Backgrounds can suggest Personality temperament, quirk, ideal, and flaw. A "Between Two Worlds" Background in its list of personality suggestions can mention as a Flaw, something like has difficulty forming deep friendships with Human characters because of the pain of loosing ones own Human family member. And oppositely. Mention an Ideal to make a point to be kind toward Humans in honor of a lost family member.
but the fundamental reasons for the initial disconnect apply to all half-elves equally, then how that goes on to affect their personality is a matter of BTIF
A Human/Elf whose parents, grandparents, great grandparents, and older living ancestors are ALL Human/Elves wont feel that kind of disconnect, since no one close has a Human lifespan.
That kind of disconnect is cultural − depends on which culture the Human/Elf comes from. Even then, different Human/Elf individuals within a same culture will have different experiences and different personal feelings about it. For example, if the Human parent might be a mage who acquires an extremely long lifespan by other means.
i'm not talking about multispecies rules though am i.
This stuff feels cultural. Background has the design space to represent these things well.
Note. The Background descriptions for a specific fantasy culture can fall into the same trap of habitually echoing reallife racist stereotypes. These reallife stereotypes must be avoided within Background as well as within Species. Fortunately even if the stereotypes appear accidentally in later supplements to 2024, at least the fact that a society is complex with different subcultures, contributes different experiences within it and helps cultural Backgrounds avoid the ubiquity and homogeneity of a stereotype.
