320ppm isn't bad at all. I've kept wild hypancistrus and peckoltia for extended periods in 460ppm with no problems. Definitely age/agitate the new water. It should only take a few hours if you use a pump, longer for airstones. I don't bother heating new water if the temperature is within 10F, but a lot of people prefer to heat it.
If you feel strongly about using RO, you can ditch the DI unless there's something specific you're trying to remove. The 10-20 increase in TDS just isn't a big deal in this case.