I need to work on a similar system as this when I get my larger tank up and running. Whilst buckets are a pain I've coped ok with them so far.
2 things are bothering me!
I can siphon out of the window onto the shed roof, from where it will drain into a butt and can then be used for the plants (if it ever stops raining!) therefore don't need to power the process. BUT if I'm cleaning the gravel as I siphon, which I usually do, I won't know if any little fish have gone with the poop! I confess I've never had to rescue any out of my bucket, but I still always check before tipping out the water!
Also adding such a large amount of water treatment to the existing 50% water level before topping it up? Is that not a problem to the fish? I faffingly add the small recommended dose to each and every 10 litre bucket I fill!!! - pain in the ... But I thought that's how it had to be done!
BTW - if you're gravity siphoning, you only need to suck the water over the highest point, not the whole length of the hose, for the water to start flowing. With my shorter hose and bucket method, I dunk the whole hose in, thumb over the draining end and take it down to the bucket - voila, water flow and not a drop drunk!