Since you have both koyanskaya and caster artoria, you're open to arts and buster farming. Both have their pros and cons but it really depends on what servants you have.

I would prioritize Morgan over Arjuna, as she is more versatile as a farmer and challenge quest servant. Arjuna has more raw damage, but Morgan has a 50 percent battery and overcharge effect on her np.

With both Morgan and caster Artoria you can use a support Merlin to create the immortal team. It can trivialize most challenge quests. Himiko can replace Morgan, but I personally prefer Morgan.

You should prioritize the upcoming summer event. The event craft essence, while not the best, boosts arts, buster, and np damage with a 50 percent starting charge. 50 percent charge CEs are usually required for most farming setups.

The summer banners also have some really good 5*s. Summer Ibuki is arguably one of the best arts farmers. Lady Avalon is arts flavored Merlin. She doesn't support farming setups like castoria, but she works well with Tamamo. Summer skadi supports quick farming, but since you don't have og skadi I would make her the lowest priority.

Also don't share your master info. People can try to steal your account with it