Distilled and RO is almost, or actually, the same. It should read like 0,01mS it depends a bit on your RO hardware. Even rainwater, there's almost no ions or nutrients in it. Just pure water. This makes actually good plant water as there is 0 bicarbonates or natrium chloride in it but it'll lack the calcium and magnesium of regualr tapwater, so that is where Cal Mag comes into play. Some tapwater is ok if it's soft, and even medium hard tap works in some setups, like organic soil. Hard tap water needs to be dilluted down and pH to, at least, neutral, to be of any use longterm.
What I ment is you shouldn't stick your pH pen in distilled/RO/deminerlized or pure rainwater. Use your EC meter beforeahead, if water reads at least 0,2 EC then the pH probe will not suffer.
RO is ok for plants if you have these types of "supersoils" which already contain all plant essentials including Calcium and Magnesium. It remains to be seen if your mix actually fullfills this.
Once water hits your substrate, it will dissolve some salts and these will make it also into the drain. The bacterias in wormcastings also work on this. It's helpful if the water is at roomtemperature and not cool. My drain EC reads about 1,2 mS and can go up to over 3 in fresh organic soil. That's some false reading due to organic molecules therein. I've had readings with some fresh potting mixes at EC 8 then, plants wouldnt grow much and even showed deficiencies (!). Cannabis can adapt to anything in between EC 1-2.5, above that be weary. Correct pH is way more important, 6-6.5 in your case - but it's also important which pH goes in. Your substrate pH is usually in between these 2 measured numbers. I usually (organic soil,
Biobizz) irrigate pH 6,5-7, and get 6,2-6,5 drain reading, and then it's good.
The general problem with lacheate probing is the actual amount of water used can dillute your numbers measured. So you need to stick to a somewhat reasonable volume used and keep that routine. Like, first slowly saturate the medium, then drain out like 20-30% of potvolume. That's why I like slurry shakeprobes as these can be set more accurately. But they require more time and preparation. Drain is quick to do just for controlling here and then.