I find the analogy of muspelheim to the sun to be more encouraging than the goddess Sol.
Then again, I know i have several farfetched opinions on Norse mythos' which likely have no place in a serious discussion... such as 'jotuns' referring to a prior culture, and the Aesir/Vanir descended from their religious beliefs, hence the jotuns as antagonists and as inter-marrying protagonists in the stories.
Anyway, your point is well taken on Sol and Luna's minimal mention in surviving documents.
Yet, there is a surprising amount of indirect referencing...
[
www.mundilfari.org]
seems a pretty good compilation with citations.
Most revealing to me, and why I hold the opinion that Muspel should be considered the sun analogy, is the requirement on Sol to hold the shield Svalin before her to shield herself from the sun lest she burn up... yet, in other references, Sol is the Shining God drawn across the sky in a chariot (echoing AE perhaps? nah, just echoing the solar deity in general)
The sources we have, reaching back to before Odin, reveal a self-contradictory stance. That's likely due to the reconstructions and translations and sheer distance in time we have, but it renders any discussion such as this one mostly speculative, or, at best, interpretive.
heh, but that's where the fun in comparative mythology comes in, right?