Wildshape's restriction might refer to the "cast a spell" action, but you use the "attack" action for radiant sun bolt. The 2 points of contention are:The Way of the Sun Soul specifies the radiant attack as "Ranged Spell Attack." Dunno if that counts as casting a spell or not.
Mu.And since I brought it up, what's the best build for a Barbarian/Sun Soul Monk?
So, AC12?And for the record, monks don't have to go DEX. Yeah you need 13 for the multiclass, but if you're going Barbarian/Monk MC then I'd just stick to building STR and take a monk weapon that can deal STR damage.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.