I find blackcurrant Dioralyte works quite well in warm conditions - unlike some sports drinks it is easy to drink without feeling sick.
If you want to increase the amount you drink you could try a CamelBak style pack. They make drinking easy compared to bottles and hold up to 3 litres which would keep you well hydrated if you fill up at each control. Not sure whether it would be comfortable to wear one on a 600 though.
I'm gonna actually suggest that a camelback style bladder system, has a weakness. You can't easily tell how much you have drunk. You may think you've taken a sip every few minutes, so that must be loads, but then find you've still got 2 litres in the bladder. This is more or an issue when it's in a frame bag, or when there is other stuff in the backpack when back mounted.
Personally I like a 1L bottle. I carry 1 or 2 of them, depending on temperature.
Last summer I did.3 200k DIYs in 6 days, in ~34°C temps. There was minimal breeze, next to no shade, and little cloud cover. I wore a white helmet, a short sleeve merino jersey, shorts, short socks, and what felt like half a litre of factor 50... On each ride Iost count of how much liquid I took in at 6L. Mostly water, with some coke and Fanta at cafes. On the hottest day of the 3, despite the constant input of liquid, I went 8+ hours between toilet visits. I planned my rides based on available water sources (did I mention the drinking water POI overlay of Osmand?), Literally riding from drinking tap to drinking tap. Usually 40-50km between each. Consuming at least 1l during that distance.
My clothes had a coating of salt crystals by the end of each day. There were times when having filled my bottle I had a brief moment of trying to decide, do I drink it, or pour it over my head. On one occasion where I did do spray it over my head to cool down, it just washed all the salt into my eyes, which stung... On only one of the rides did I wake up the next day dehydrated. Which was soon fixed. I didn't take any fancy hydration salts, but did have a packet of ready salted crisps each day, as well as a portion of salted fries.
I'm of the view that the trick is to drink lots, starting before you start riding, to keep a good idea of how much you've drunk, and if in doubt, drink.
J