Iv always put my water through the RO and into the tank, then run it through the DI as needed.
To be honest that's the way the system was when I bought it and have never found any reason to change it.
Advantages are you can bypass the DI and use RO water for conservatory roofs and fascia cleaning on properties that don't
have access to an outside tap, this will save on resin use.
Plus if your RO water happens to rise for any reason then you don't end up destroying your resin as well.
Disadvantage is you will have to carry the DI bottle in your van and if your tank TDS is still on the high side then you will have to
do a few TDS checks over the day just to be sure your resin hasn't failed.
At 30ppm I wouldn't be too concerned.