I am curious about how a Celestial warlock/Sun Soul Monk Multiclass would work.
From Xanathar's Guide to Everything's Celestial Warlock's feature Searing Radiance:
when you cast a spell that deals radiant or fire damage, you can add your Charisma modifier to one radiant or fire damage roll of that spell against one of its targets
And from Xanathar's Guide to Everything's Sun Soul Monk's feature Radiant Sun Bolt:
You gain a new attack option that you can use with the Attack action. This special attack is a ranged spell attack [...] Its damage is radiant [...].
I am curious if RAW or RAI you would be able to add the charisma modifier to the Radiant Sun Bolt, since you are not technically casting a spell, just using an attack that is considered a spell attack.