As you've already quoted, Martial Arts lets you make an unarmed strike as a bonus action. Radiant Sun Bolt is not an unarmed strike.
Furthermore, Radiant Sun Bolt is a ranged spell attack that can be used with the Attack action, not bonus action, and you can only use it as a bonus action if you're attacking with Radiant Sun Bolt and spend ki points.
You gain a ranged spell attack that you can use with the Attack action. [...] When you use the Attack action on your turn to use this special attack, you can spend 1 ki point to make two additional attacks with it as a bonus action. (SCAG, pg.131)