While I'd like to increase the relationship with Belle, I think it might be best to start the training off with discovering our pokemon's limits, even if that means they're too tired to train the next day. Yes, the spar also tests their limits, but there's no way they can beat Belle right now. I'd like for them to be able to put up a fight. Plus, the spar has the least amount of training effectiveness, compared to the other options.
Haru is there for 2 weeks so maybe start off with the special training and then switch to normal training with Belle? We can then spar with Belle at the end of the first week and see where to go from there.