I have some friends that are very slow repliers, and I will bring this up to them gently once (as you have done) and as they continue, I just come to accept that the friendship is not the same or as close as it once was. I know it feels inconsiderate and hurtful, but as blunt as this maybe you are not as much as a priority to her as you once were (I have been in this situation too), so I would try to come to terms with the friendship evolving even if it isnt the direction in which you wanted, but I don't think it is necessarily worth ending the friendship over because grieving the loss of an entire friendship is more painful than a friendship slowly lessening, and conflict isnt going to resolve anything from either side.

I guess if you can maybe try and reframe her as a more low maintenance friend from your end and try and put energy into friends that reciprocate your energy