I forgot:

Another option is to use a stored procedure alone, the thing is that
there are 20 calculated fields and 300 "direct" fields.

I am wondering if a stored procedure of 320 fields is a good idea? I
think that it can penalize the performance because it always manages
all fields, in a view only the selected fields are managed or
calculated.

Until now, a view can manages 300 in a very satisfactory way.