Depending on how much you want to answer: 2, 3 and/or 10 for the worldbuilding questions? (Also, though off topic: I love your art style)
2) We all know about weddings and marriage, but are there any ceremonies that symbolically / legally / magically officialize a different type of relationship in your world’s culture? (Adoption, apprenticeship, friendship, etc.)
The Tahen peoples have a weaning ceremony. In Tahen cultures, hatchlings are raised by their parents until they're weaned & have shed their neonate duvet, a little before one year of age. When they reach this age, the duty of childraising is transferred from the parents to the grandparents (traditionally the maternal grandparents, though this varies). At this point, the grandparents hold a feast to celebrate the arrival of the children in their lives. There are gifts to the child, and theatrics where the parents pretend to be reluctant to give away their children while the grandparents reveal gifts and make increasingly grand promises for the children's future.
Among Ranaite Tahens and other Tahens where maternal grandparents no longer have automatic precedence, these feasts may also be rather tense as all sets of grandparents use them as a way to show off their willingness and ability to care for the children in a bid for custody. Of course, in most functional families, this has been orchestrated beforehand and the grandparents have already agreed on custody details for early childhood - as they get older, children typically move freely around the houses of all their grandparents.
3) What’s a rule or social norm that is widely followed in theory, but in practice everyone knows it’s not a big deal and breaks it all the time?
Technically this is more of a "what's a rule that everyone breaks all the time" answer. Among Tahen cultures as a whole, writing and reading are revered, and the dominant religion holds the letters of the alphabet as gods. Writing is therefore both mundane because everyone does it all the time, and sacred because letters are gods.
About 50 years prior to story time, there was a fad among Ranaite Tahen teens to deconstruct the Old Tahen characters for their names, tallying the types of strokes, selecting the most common ones, and assembling them into a unique glyph. They would then use these glyphs in place of a formal signature. To the Tahens, this is borderline sacrilegious as the Old Tahen alphabet is the basis upon which the current (godly) alphabet was built, and also a perversion of the act of writing. Unfortunately this trend caught on and was there to stay, and to this day it is common for almost everyone to use a personalised glyph in place of / along with a formal signature, despites these having no legal tender and being officially prohibited on formal documents.
There are even semi-formal glyph-taking ceremonies, originally organised by teens among themselves during graduation parties, but now often sponsored by their families as well. There are efforts to formalize and legalize glyph usage, but these are quite controversial.
10) What’s a fun fact about your world that you as the worldbuilder are dying to share, but nobody ever thinks to ask?
Well tbf people don't really ask me that many questions, and I'm pretty good at talking about the things I want to talk about in general. That said, I wish more people knew about this silly genetics thing I made a few years back: pigmentation genetics was a really big interest of mine before I got obsessed with textiles!
