helping with sprinkler conundrum
Yes, it's a fine idea to add another valve and manifold location and to convert that long section of red so that it's an always-on main line rather than just a zone 2 feeder.

Your desire to add a separate zone for the garden, rather than make the lawn zone water the garden too, is a good one for several reasons. For example, gardens and turf grass have different watering requirements; and broadcast watering over the garden wastes water and supports growth of weeds all over the garden.

When running wire to the new valve location remember that you need a conductor for every valve, plus one as a common, plus one or two spares for when "it would be nice if..." happens later. ;-)