I like your thinking here as a labor saving idea. Remember, if you
continue this program you will always have at least a tiny bit of the
original fluid in there. I don't consider that critical though.
Assuming the 1.5 liter tank is included in the 6 liter capacity then
you are draining 25% and keeping 75%.
First change = 75% left of original.
Second change = .75X.75 or 56.25% of original.
Third change = .75X .5625 or 42.19% of original.
So at the end of the year you have less than half the original fluid
left.
In the case you describe I think it makes sense. Eventually you have
almost none of the original.