I bought three Eheim double tap connectors, one is used on the filter return for other maintenance reasons, but the other two are for water changes (I also do 50l per week from a couple of 25l containers.
Basically, one tap connector is on the filter inlet pipe, the other is separated and attached to opposite ends of a long length of hose, about 1 metre. To siphon off, I switch off filter, close both taps, break the connection and add in the tap end of the spare hose that connects to the supply from the aquarium. Stick the other end in a bucket, then open the taps. Syphon is ready there, it just drains straight away. Fill the bucket, jus close the end tap and reopen after emptying the waste bucket.
When you refill, just close up, take off the spare hose and switch ends so the other end now connects to the inflow to the filter and the other end goes in to the fresh water container. I stand this on a small side table or chair so it's higher than the filter. Sometimes takes a little priming, but essentially once started the filter just pumps your fresh water in to the tank.
Someone else on here suggested the idea to me as they use it too. Works really well.