The therapist may be concerned about the "psychotherapist-patient privilege" under the California Evidence Code, and which she has to protect. If your ex is hostile to your getting expanded custody rights then the therapist is also likely concerned about a complaint to her licensing board from her if she goes to court voluntarily.
Your idea of having her subpoenaed to court is good, but she may not be able to testify about anything your child said (or even did) during their sessions if the Mother won't waive the privilege. The court can limit her testimony to just what was discussed between the two of you, and her observations of your parenting skill and bonding with your child. If the court orders her to do so, the therapist should be able to say that her supervision is no longer necessary and that you are ready to go to the next stage of the visitation plan.
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