I would fit a tank connector to the dwc bucket and a second bucket the same size. When it comes to adjustments or nutrient changes, link the buckets with hose and raise the dwc so the water drains into the empty bucket. Make the adjustment or nute change and raise the bucket to refill the dwc.
Using john guest pushfit tank connectors will allow you to unplug the 2nd bucket with a minimum amount of fuss if required. If the connecting hose is clear it can double as a water level indicator / water topup port. Fit a pipe clip to the top of the dwc bucket to keep the hose upright.

Are you using liquid ph adjuster? I would think that the mixing action from an airstone would cause enough agitation to mix two liquids together. You might have to give it a bit time before you can check it accurately. I have a level indicator hose and I use that to drain water, check nutrients, adjust ph and put the air into my buckets. ( rons bubble buckets on youtube ) I just put a bit of ph down into the indicator hose, reattach the air line and check it later.
kevin

Thanks for both responses. Both are great ideas. Thanks for also pointing out that air stone agitation would mix it over time. I will add a little and wait a few hours. Thanks again for your insight.