TDS 400 or less is what I target for generally. Your RO 8ish pH at the start doesn't make sense to me with with your kH/gH, TDS readings. Do you age your RO water? Regardless, your CO2 system is bringing your pH down to 6.8 is fine and folks have been successful with 8ish pH.
I would to a test of aging your water so see how much your pH reading changes; otherwise, I don't see any issues here unless there are sudden fluctuations in these readings.