Looking great Fred.

Yes, you'll certainly find that the water from the pond will be very rich in nutrients for vegetables and other garden plants etc.

I always used water out of my fish tanks for irrigation in the garden and for pot plants. Also in my (separate) aquatic plant tanks.

Partial water changes are a must to maintain water quality in a 'closed' aquatic environment if you want to have a decent stocking level, so being able to use the old water productively is a bonus.