Meanwhile I was researching possible features for my future Instagram planner, I figured out it's not really possible to create one due to Instagram API limitations. I could use third-party services to do necessary stuff, but it's overcomplicated.
Then I thought I could create the same tool, but for LinkedIn. I want to keep the simplicity of the basic idea: just posts scheduling for the MVP.
So, the lesson is: do an investigation of technical implementation properly, especially if it concerns third-party APIs!