The last part with some tips and ideas.

• As an exercise in photography, it is a good idea to explore some subject matter or technique that one does not feel immediately drawn to.  A Japanese friend would load in the morning his camera with a 12-photo film and force himself to go out and photograph. One morning he would set out photographing shadows for 15 minutes. As a textile artist, he saw it as an exercise to focus his mind and prepare himself to paint on various textiles. Obviously, he could have been particularly good as an art photographer too.
• Highly recommendable for those that take photography seriously are two small, yet outstanding books.  Roland Barthe’s, ‘Camera Obscura’ and Susan Sontag’s ‘About Photography’. They are must-read books on the philosophy of photography, leaving deep impressions on anyone taking photography seriously. Sontag’s book is particularly complete. Both are available in translations and in pocket editions.
• In my view, two completely unrelated subject matters to photography will augment one to become a better photographer.  They are most important as portrait and event photographers. One is negotiation, the other is some insight into psychology. Getting a good vantage point is often the subject of negotiation and essential for event/photojournalists.
• Looking at a portrait, intuitively most of us will be able to tell a lot about what was going on between the photographer and the sitter. Perhaps too complex and elaborate to go deeper into the subject, in this brief treatise.  However, experience demonstrates, time and again, the importance of both subject matters. Selecting the photos in collaboration with the sitter or model can make for an interesting learning experience.
• Selectivity is most important. Few photographers will exceed 3 to 5 good photographs, out of 100. Often I involved the model or sitter in the initial selection.  That is particularly important when dealing with movement, like ballet. Then selecting the very best of the three photos is an interesting exercise
• On Instagram, we often get a very good insight into that lack of selectivity. The best photograph is shown first and some ‘lesser photos’ are added, usually ignored by the rapid ‘clicker’. Too many photographers fall in love with the models or their own photo(s) Or when some see their first ‘professional’ made photo.  The result is often beyond rescue... the inability to take a step back is important in all processes.
• Having a reliable critique source is good to have. Most excellent critique sites on the internet have gone. Pointing out the good points and those in need of improvement is important. There is some critique on fotocommunity.de or similar, but it is mostly absent in a constructive way. Setting up a group on Facebook or similar is a clever idea. Sometimes a photographer, just like an IT technician, needs to solve a problem and can solicit the help of a colleague or get feedback.
• Learning about post-production, using Photoshop or similar software is essential in the digital format. The use of masks and layers and other techniques can be found on the internet, but for many a classroom may be a better setting.  In classrooms, we also learn from others. Portrait photographers should have good insight into anatomy and be able to apply the right brush strokes.
• Lastly, about printing photos. Having a good printing lab or printer is important in the selection of paper and prints.  Do not believe that using the same paper and machine will produce identical prints from digital files. Humidity, and monitor and machine calibration are always factors. If one needs three identical prints, have them made at the same time. The devil is, as it often is, in the detail.
• Recommended course: Seeing Through Photographs, a free course offered via Coursera on the Internet. It is a short course and explores various approaches to photography, including the use of non-camera imagery. There are plenty of good books. Those mentioned above are good starters.