GR was (apparently) broken by changes in mass; suppose instead that animagi convert between magic, energy, and mass in an analogous manner: When in a smaller form, they are more magical, and vice versa. Brooms convert magic to a combination of energy and/or something which is anti-mass but not anti-inertia, and/or something else which is not directly analogous to anything in GR.
The time turner then remains no harder than apparation combined with future prediction; future prediction is easy enough that it’s provided in novelty drinks.
GR was (apparently) broken by changes in mass; suppose instead that animagi convert between magic, energy, and mass in an analogous manner: When in a smaller form, they are more magical, and vice versa. Brooms convert magic to a combination of energy and/or something which is anti-mass but not anti-inertia, and/or something else which is not directly analogous to anything in GR.
The time turner then remains no harder than apparation combined with future prediction; future prediction is easy enough that it’s provided in novelty drinks.