"Is there any known method to calculate the (or a) fundamental value of Steem?"

• No not yet but the fundamental value could be considered all active users + the amount of transactions performed on the network.
  "Why is Steem underperforming other cryprocurrencies? (If it does; it’s a question of comparison methods and time frames again.)"
• The Steem reward pool creates a 8.5% inflation per year and thats creates constant sell pressure on the price of Steem. If somebody wants to calculate his investment on Steem you also have to consider all the rewards that you have gained in 12 months curation, author and Bot rewards.
• How will Steem handle the competitors and how competitors will perform? (Tron, Lino, etc.?)
  There is no way to handle competitors because its a decentralized network. Every network has its own advantages and disadvantages and are mostly not comparable to each other. Thats why its more important to grow the community and bring in good developers who creates the Apps.
• What are the differences between this competitors? Are they really competitors?
  This is not a traditional market where you can compare companies with their earnings. Steem is not only blogging plattform it could be instagram, product hunt, crowdfunding, steemmonsters card game etc...
  so the only measurement is again the amount of active users and transactions performed on the network.
• When will Smart Media Tokens be ready, what importance will they have and how will they affect Steem price and Steem economy?
  
  Thats why SMT release will be delayed...hopefully we get it stil in 2019.
  But still projects like Utopian.io and Steemmonsters already show that you can achieve a lot without SMT. SMTs are one of the major milestones that will be implemented in the core Steem code. @ned from Steemit.inc just announced that he had to lay off 70% of his staff because of funding issues.