Hi and Welcome
I beleive the pond is too shallow and will heat up and cool down rapidly, this can stress fish. A deeper pond would be more beneficial. I'm thinking 50cm+. It depends on how much light the pond will get also.
Acorus is a good bog, marginal plant. Asthetically pleasing IMO.
Or you can add oxygen weed, duck weed or any other cold water plants. These plants are easy to grow and have very little requirements. Or lilly pads? :thup:
Adding things before plants: if they are going in the water just chuck them in. I they are going around the pond maybe a fish safe fertilizer will help them grow. Maybe unecessary :dunno:
Adding things before fish: A chlorine remover is probably needed depending on which fish you wish to add (or you could use rain water which won't need a chlorine remover). A filter could be useful, especially if you would like to see the fish :slfg: . You may need to do some research on which type of fish you would like to keep. You could expand to frogs and other animals, but remember they all have certain requirements. :smln:
Hope you enjoy your time here,
Matt