You can never disconnect the autopilot in the Airbus unless you pull circuit breakers to disable computers and put the aircraft into direct law. You can put the aircraft into control stick steering. This will work at any altitude. Minimum altitude to switch to CSS will be governed by your airlines SOP. My airline uses 50 feet below minimums or normally 150 feet on a CAT 1 approach. Disconnect can be down to flare if the approach is not satisfactory. I solve the issue by always flying cat 1 approaches in CSS to maintain currency in the Airbus version of hand flying. Why anyone would couple a cat 1 eludes me. The airplane can fly it just fine. Pilots need practice for the day things go wrong and you must fly the aircraft.