I have done water changes with a hose, direct fill from the tap, and I just add dechlor into the flow of incoming water. If it is a large water change, or a 'constant change' sort of event I know how much water is flowing from the hose (I have timed it) and I keep on dosing dechlor to stay ahead of the incoming water.
I have done really large water changes with the 'constant change' idea and dosing dechlor periodically with no problems.
I drain the tank just a few inches, then start the water coming in. Temperature adjusted at the tap. Then I keep on draining the tank, but refilling at the same time so the water level stays pretty close to the top, or a few inches down. Every few minutes I know to dose X amount of dechlor. I know the 'new' water is mixing with the 'old' water, and what I am removing has a certain % of old and new.
At the end, tank is topped off and I can test for chlorine and ammonia (Chloramines) and dose a bit more dechlor if needed.
Fish can tolerate a certain amount of chloramines. You can very slightly under dose dechlor and not have problems with most fish.
I top off tanks up to 10% of the volume without dechlor.
I can be a bit casual about the timing of when dechlor hits the tank.
Barbs (Including Goldfish) seem to be a bit more sensitive, so I usually err on the side of overdosing the tanks with these fish. Not by much, though. A severe overdose can lock up oxygen. Read the label: Most dechlor can be dosed at several times the basic dose to help with ammonia or other toxins, but there will often be a caution about maintaining good aeration.