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Don't use ToString (and especially, don't use a single argument ToString). Instead, just use expressions instead of strings:

Subscript[θ,n]==π/2

Addendum to address updated question

Again, don't use ToString. Here is how I would create your legend:

θ = Pi/2;
Style[HoldForm[θ] == θ, 20]

(Note that legends in plots are by default displayed in TraditionalForm, so that the double equal symbol gets rendered as a (long) single equal.)