Not sure if this is an appropriate question here. The thing is that to truly learn a language, you'd usually need to express yourself via writing and speaking, and find somebody knowledgable about the language to provide feedback so that you gradually use the language more authentically. However, for a self-learner, this seems to not be easy. One way I can think of is to maybe find another person learning your language to form a pair, however I'm not sure how reliable that approach is. Are there any suggestions on this topic?
I have had the same difficulty w/developing "productive" language skills (writing & speaking), as opposed to "receptive" language skills (reading & listening).
Productive skills often aren't as strong a focal point for formal courses, and as you've found can be difficult for the independent learner. You need access to native speakers (or at least advanced learners) to avoid getting bad advice, or to get any feedback at all. It's also easy to get in a rut where you have a few memorized/frequent phrases surrounded by large gaps in vocabulary.
1) Use social media
Do you have any native speaker friends on Facebook, Twitter, hobby/work-related chatrooms or message boards? If not, seek out those communities (like this one), and practice. Rewrite your question above auf Deutsch & wait :) Try to ask any future questions here in German too, just for practice.
2) Write posts or responses to online articles in your target language.
If German speakers are anything like English speakers, they'll LOVE to incessantly correct poor grammar on the Internet, so you're sure to get some feedback.
Bonus: this provides a practice platform, potential feedback, and access to modern colloquial written speech you won't find in textbooks anyway.
You may already be reading/following topics that interest you - take the extra step & write about them. Also perhaps create a list for topics you're weak on & seek out related content so you can practice and broaden vocabulary as well.
3) Blog or keep a journal
Whenever you read/watch/listen to something in your target language, write a summary of it and your thoughts about it. Do this on a regular basis to get more comfortable pulling words out of your brain & onto paper/screen. Write about a niche topic (work, hobby, etc) that interests you, or make it a habit to write about different things to stretch yourself.
The feedback you get may be slim, but the process will be helpful, and once you do find an audience you'll have written content ready for them to help you with.
4) Tutors, personal ads, language exchange, local German groups, etc
As you said, having other learners critique your work may not be ideal if that's your only source of feedback, but it's better than nothing. If they're more advanced, they can help you. If you're more advanced, helping critique their writing forces you to think about and hopefully improve your own.
5) Remember "pen pals"? Is that still a thing?
- Previous post: How to find pen pals?
(Just be careful sharing personal details, as w/any online contacts. Don't send any money to Nigeria or whatever...).