Sounds like you have a pretty good grasp on decks or have at least tackled some before. It makes it nice when they still have the old stain can to look at. You might already know it but, for future reference, one way to tell waterborne between acrylic is with denatured alcohol. Put some on a white rag and rub the wood. If the rag brings up any color you have waterborne stain.
I usually mix sodium percarbonate at 8 oz per gallon. You can try 10oz but it might not all dissolve. Mix into hot water. It does have a shelf life so has to be used the same day. I usually mix mine the morning of and right before I leave for the job. Make sure your container is vented. I use a 5 gallon bucket with a plastic fuel filter poked through the lid.
I usually don’t brush the vertical surfaces unless they’re covered in algae. You’ll see the sodium percarbonate working right when you spray it on there. Sometimes you don’t even need to scrub. Looking at the deck above it might not even need it.
I mix oxalic at 8 oz/gallon too. Let dwell 5-10 minutes and rinse.
If you stain a lot of decks I would highly recommend investing in an airless sprayer. I’ll never do another deck without one. It might take an hour or two to tape and hang plastic but you can fly when staining. This 12’X24’ took me about 2 hours to spray 2 coats and that includes back brushing each coat.
Not sure why I made the pictures backwards here.
I don’t stain a lot of decks but I like to do them every so often to mix things up. Plus, there’s good money in them.