Hi Cricket
There are some potential side effects from the treatments.
Peroxide needs to be inserted slowly and may feel very strange when it starts to fizz. It can also make some very embarrassing sounds as the air escapes. There is a small chance of irritation.

Garlic can sting so I suggest that you only make 1 small cut in the clove the first time you use this. Increase the number of cuts as you get used to the feeling.

Chlorhexadine is used widely in hospitals to kill germs. It has a slight chance of causing irritation when inserted into the vagina.

Personally I would suggest the garlic for its long term protection. I’m not sure if the other treatments offer this.
My second choice would be the Chlorhexadine as there is a very slight theoretical chance of an air embolism from peroxide due to its ability to release air when it comes into contact your skin.