Here are my thoughts on your opening questions:
* “Is Solstice primarily a rationality holiday? An EA holiday? The broader secular community?”
Empirically and normatively, rationalist.
“How essential is the journey from light, into darkness, into light?”
Pretty darn important. As you ask at the end, I could see an occasional or one time “journey from light, into darkness, and that’s it” story. It would make for a good “final episode” before the world ends. I’m reminded of the final episode of the sitcom Dinosaurs, where due to out of control technological change, an ice age ensues, and the main characters huddle together for warmth as they slowly freeze to death.
“Is it okay to have a Solstice where we don’t sing Brighter Than Today?”
No. Except maybe in the “from light to darkness” one, where we could sing a version with altered lyrics.
“How important are singalongs vs speeches?”
Singalongs are incredibly important. Speeches I could do without.
“How important is it for singalongs to sound polished, vs for them to feel like an organic part of the community? Is it appropriate to pay professional musicians?”
Organic part of the community: incredibly important. Polished: of negative value. Paying professionals: I would prefer not.
“How important is transhumanism or x-risk?”
X-risk: pretty important. Transhumanism: I think the importance of this varies with how much people think it’s a genuine light of a new day that could save us from x-risk.
“Is it good or bad to change lyrics over time?”
Gut instinct says bad but I could see arguments for it being good in certain instances. But I’m the kind of guy who still gets annoyed at Church Latin’s pronunciation of “v” and “c” etc in Adeste Fideles.
“How important is it to celebrate Solstice on literal astronomical Solstice? If you don’t, why are we calling it Solstice? Is it important for the name to be clear?”
Ideally it would always be on the literal Solstice but scheduling is important too. People ought to be able to actually attend.
“Is it okay to have one solstice someday with a ‘bad ending’, where instead of climbing back out of the darkness hopefully, we just… sit with it, and accept that maybe it might be what the future holds?”
Yes. See above
This is the part I care the most about. If I wanted to hear professional musicians I would go to a concert. At this community holiday, I want to hear, and participate in, communal singing. I don’t want to feel self conscious about not being a very good singer. I want me and everyone else to get swept up in the moment and the song. I can recall two different Solstices I went to, one in NYC had some technical issues and wasn’t super duper polished and which everyone sang together in, and one in the Bay was much more polished and fancy and professional and had well trained musicians singing while mic’d up. I left the former with a powerful sense of community and a sense of having undergone an important emotional journey. I left the latter with a sense of embarrassment at myself for having attempted to participate in the music, like if I had caught myself singing along at the opera, and frustration at not having gotten the emotional catharsis I wanted. I found myself thinking “maybe Solstice isn’t for me anymore”.
I genuinely can’t remember if I’ve been to a Secular Solstice since then, but I have sung Brighter Than Today to myself and been overcome with emotion and cried.