Are there any other tools than Adobe Photoshop available that allow to edit PSD files that contain smart objects? (I checked GIMP but it doesn't support smart objects)
At the moment I don't believe there are any, but Affinity has it on their list of features to implement. They're both (Affinity Designer [AI Replacement] and Affinity Photo [PS Replacement]) under development and the creators have been doing an excellent job adding new features and fixing bugs regularly.
This is the most recent thread on Smart Objects I'm aware of: Photoshop Linked Smart Objects on Affinity
I've tried the software but not with Smart Objects to say how well it currently works or not.
I found a online editor which supports PS smart objects as a free alternative to Affinity: https://www.photopea.com/