Cal here from Shopify, thanks for reaching out to us. The 14-day free trial is the current limit on trial time, although we always aim to help merchants with store setup, and we can extend that for a while longer if need be and if it is requested.
However, we do currently have a Pause and Build plan which may suit you! It will require subscribing to our Basic Plan (29USD), and then downgrading to the Pause and Build plan which is 9USD a month. This will convert the difference in the two plans' prices into subscription credits, which will be used in subsequent invoices.
Let me know your thoughts!