Yeah, I use brewers friend. But I am also a greedy bastard and want my kegs full. First Charlie Papazian recipe I ever did was an imp stout and I halved everything and made 10L. It was delicious and I instantly regretted not making the full batch. Think you said earlier that 15L is the max you can brew in a corny. I could live with that if the beers are of a higher standard.I find it’s very straightforward to scale batches to whatever volume you like with software like BrewFather or BeerSmith.
Gravity reading are as you said just poured with a picnic tap.