For me it's hard to form an impression of Selûne that's particularly distinct from this...
https://en.wikipedia.org/wiki/Selene or this... https://en.wikipedia.org/wiki/Luna_(goddess)
She's basically a portmonteau of two more or less identical Moon goddesses from antiquity, with the little circumflex diacritical mark to make sure we catch the pronunciation in the FR version. The association with wolves and witches is also just sorta lifted straight out of Greek myth, with the Thessalian witches angle and such. The root word there means "bright" literally. So light would make a lot of sense for a domain portfolio to me, even if the books stress Knowledge and Life. Also Time I suppose in a pretty important sense. Selene had the epithet Mene, which was both a name for the Moon itself and a calendar term for the Lunar Month. 28 days, so a year divided into 13 months, before intercalary stuff was more widely adopted. Also a tripartite year with 3 seasons rather than 4, as we move up from the subtropics to the temperate zones where the winters are more extreme. So sorta comparing the reckoning of seasons in for example Ancient Egypt (3 seasons based on say the flooding of the Nile river or harvest times and such) compared to the later Roman reckoning described by Varro and Pliny. Roman religion preserves both the Trinitarian symbolism and the later 4 part division, which is basically symbolized by the cross later on. Like that's pretty much the nuts and bolts on that symbol, especially if it's placed in front of the Sun.
Shar I feel is somewhat more complicated. Her closest analog in the same mythos to me would be the Greek goddess Lethe, who is the personification of oblivion and forgetfulness and loss. Also the name of the river in the underworld whose waters would make the souls forget. The dark waters (contrast to the river of memory, Mnemosyne). Here's a depiction of Lethe from Gustave Dore's illustration of Dante's canto...
https://upload.wikimedia.org/wikipedia/commons/e/ea/Gustove_Dore%2C_The_Divine_Comedy%2C_Paradise%2C_plate_115%2C_Dante_and_the_River_of_Lethe.jpg
To me that's sorta Shar right there, like in whatever aspect of her is meant to be somehow appealing and alluring.
And of course Hekate, who would be a bit more on the nose when the veil gets dropped, for like the monstrous aspect rebranding. Again thessalian vibes hehe
https://en.wikipedia.org/wiki/HecateThe top image on that page, the Hecate Chiaramonti could easily be in the Gauntlet of Shar and that would totally seem to fit I think for the BG3 riff on it. The name Shar is kinda curious, I think it comes more from another mythos, sorta grafted onto the Lethe/Hekate idea but made more manichaean. So the whole light/darkness thing maybe coming in from that for her counterpoint.
To me Shar works for all the fire themed stuff coming from the light domain, because of cast shadows. So like whereas radiant damage seems off brand, a flicking candle or a lights out fireball seems to somehow fit that idea.
For knowledge and life with Selûne, I'd fold them both into some kind of idea about keeping time that encompasses those concepts. Especially cause of what the Elves make of it all and like Jaheira being on the scene.
It's hard for me to tell what's there compared to like what I'm bringing to it, but that's where my head goes. I think its kinda cooler if the deities had a portfolio standard, but also account for some adjacent domains that also work, if not exactly the go to stuff by the book. I was pretty blown away honestly when I got to the Shadowlands and the Gauntlet. It felt as if someone was taking notes in that other thread from years ago, with onyx stones that I can't find now lol, but I remember thinking a few times when playing like 'wow, they went pretty deep into it after all!'
I kinda agree though, by the story beats light and nature would make a lot of sense. In the same way that Shar has a bunch of undead necromancy type things going on with her umbral ghosts and skeleton warriors and whatnot. I was pretty into that! It totally reminded me of Jason and the Argonauts Harryhausen meets the skeleton from the last unicorn haha.