How can I make another frequency rather than one-off highlighted on the Payment Form?
This can be done using a URL token, but not on the Payment Form itself.
On the URL token in Default Frequeny field, select the frequency you wish to be highlighted. The user can still select a different value if they prefer.
Note: Default Frequency is a different field to Frequency. If a value is selected in the Frequency field on a URL token, that selected frequency will be locked on the web form and the user cannot change the frequency value.