Mixing hot/cold is a bad idea. Since your input pressure with the cold is very low, consider a booster pump. They're readily available for RO systems. You need somewhere around 60 psi for a membrane to start working effiiciently. 150 gallons total throughput is not very much & your filters/DI media should be nowhere near exhausted.
My experience is that colder water, while slowing production down a little, actually results in 0 TDS output after my RO stage after running for about 5 minutes. This is with water at 8 to 10 degrees C. In the summer, when the water temperature coming into my house ranges from 10 to 15 degrees C, the membrane manages a TDS of 1. I'm fortunate to have access to the pressure regulating valve in my home's plumbing, so I've adjusted it to provide 100 psi to my RODI. This higher pressure allows me to produce close to the rated gpd output from the 75gpd membrane at the colder water temperatures. Being in an apartment, it's unlikely that you have access to the pressure regulating valve, so that leaves you with the option I already mentioned, a booster pump. I suppose you could always complain about low pressure to the apartment superintendent, maybe they can do something about it.
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