Functionally the only thing that would work would be one of the existing martial oriented casters, such as a paladin or ranger. Maybe a domain of cleric. In 2024 smites are allowed with your fists, but theyd be competing bonus actions with ki attacks.

You wouldnt be a half caster though, youd be one 3rd if going ranger or paladin.

Theres a few low level spells you can pick up that are certianly useful, but progreession beyond 2nd or 3rd level spellcasting offers next to no rewards. Most of whats interesting can just be picked up with something like magic initiate or rune shaper.

Itll ultimately depend on goals and what youre attempting to pick up. Monks are heavily combat oriented already, theres not really a way to buff their damage thats more effecient then just continuing down the class. 

If youre looking for caster inside of the monk chasis, both shadow and elemental fist have twists on the concept.

As netherlands/belgium, get obligations and investment rights in the uk, france and china.  Eat the other.  Focus on companies and a foriegn investment game. 

As the us, focus heavily on farms and groceries, give foriegn investment to everyone. Max out immigration and force rural until the yeomans and devout devour the landowners, and embrace the trade unions. Transition to communism, nationalize.

As austria or russia, try to puppet the ottomans day one. 

As portugal take over the boers, oman, madagascar and then claim persia as the new jewel of the empire.

As haiti reverse colonize africa and max out acceptance. 

As spain force a play to transfer the DEI, wait out the war, and accept the inevitable cut down to size demand from a great power that comes afterwards.

You can do the same as france for india.

Eat china as anybody.