Ya I would just hit it with a solid 5% roundup and be done with it. Plan on hitting it each time it gets enough growth to justify spraying. The goal is to starve the plant of carbohydrates and actually kill it.
The higher the PSI isn't always the best choice. Wind can affect higher PSI since the smaller droplets are more susceptible to being carried by the wind. Also is it 5,000 sq ft of application area, or is your property 5,000 sq ft total? Everything in my opinion seems to be pointing towards the 2...