Hi vilas,
If you have to do anything else like reducing thrust etc. means you didn't anticipate the next action. It is only salvaging a bad situation and not smart flying.
Probably easy to say with your level of experience.
From FCTM the second of the Golden Rules for pilots says:
"2. Use the appropriate level of automation at all times. ....
Determine and select the appropriate level of automation that can include manual flight
Note:
The decision to use manual flight must be agreed between both pilots and must be based on an individual assessment of the pilot. This assessment should include aircraft status (malfunctions), pilot fatigue, weather conditions, traffic situation, and if the PF is familiar with the area."
Now the OP poses the question of how to handle a TOGA climb of only 300 feet. I would suggest that full TOGA power is inappropriate as ALT* will be triggered very quickly. On previous aircraft types the manufacturer fitted a reduced level of TOGA power and the easy ability to control the applied thrust - but it's not quite so easy on Airbus.
However if the OP selected TOGA initially until the FMA confirmed the change then he reduced the power below the climb gate to say around 75%, he would have more time to reconfigure the aircraft.