That is not easy.
As a general rule, I only would buy a camera you like to take in your hands. If your stomach feels like "meah" with the Z5ii in your hands, but your heart rate goes up with the ZF in your hands, that could be already a strong indication.
But with sports at 80% of the use cases, this is more difficult. Also the sport genre plays a big role here. The differences in the AF systems. You should test it beforehand.
For swimming, every AF will have problems because of the water drops flying around. As a consequence, you need more DOF to be sure that the head of the swimmer is still sharp in case the AF picks on waterdrops.
Due to this you might need higher ISO. Light at swimming events is probably not the best one. Soccer outside is not a problem. Neither for modern AF nor for high ISO.
I do not know the AF of ZF and Z5ii. But it will be likely equal or better than the AF of my Lumix S1R2.
I have no problems to take pictures of field hockey outside with that camera and the Sigma 70-200/2.8 DG DN Sports.
But the lens plays a big role here. With the Sigma 100-400 DG DN, my hit rate is not as high.
For the other 20%, you do not need AF at all. Therefore it does not matter what you pick.
Which lens are you using for soccer and which lens for swimming? Maybe it is better to buy a second hand body and invest in a better lens?
- Panasonic - DC-S1RM2
- 70-200mm F2.8 DG DN OS | Sports 023
- 171.0 mm
- ƒ/2.8
- 1/2000 sec
- Pattern
- Manual exposure
- ISO 200
- Panasonic - DC-S1RM2
- 70-200mm F2.8 DG DN OS | Sports 023
- 200.0 mm
- ƒ/2.8
- 1/1600 sec
- Pattern
- Manual exposure
- ISO 125
- Panasonic - DC-S1RM2
- 70-200mm F2.8 DG DN OS | Sports 023
- 200.0 mm
- ƒ/2.8
- 1/1600 sec
- Pattern
- Manual exposure
- ISO 100
- Panasonic - DC-S1RM2
- 70-200mm F2.8 DG DN OS | Sports 023
- 191.0 mm
- ƒ/2.8
- 1/2000 sec
- Pattern
- Manual exposure
- ISO 200