How can an advanced learner in an FL context get rid of grammatical errors?

Question

Imagine someone who learnt foreign language (FL) from the same language family as his native language (L1) for three years at secondary school and then continued studying this language at university. Let us assume that this foreign language is German (so this person's native language is another Germanic language). Then they neglect their German for 10-15 years except for occasional reading in the language. Then they move to Germany. They still have CEFR level C1 (close to C2), thanks in part to the similarity with L1.

After living in Germany for 7-8 months, they find that most German language courses for CEFR level C1 are not challenging enough and that C2 courses (if they are offered at all) have many participants who are at level B2, which decreases the effective level of these courses. So German language courses for non-native speakers look like a dead end.

They still notice grammatical errors in their own German, and that they avoid certain structures (i.e. pick alternative formulations on the fly) for fear of making mistakes. Otherwise, their speech is fluent. Native speakers (including colleagues) do not correct them. (Error correction appears to be rare among adults.) How can this very advanced learner get rid of the remaining grammatical errors in his German?

(I used German as an example, but it would work for any language, though the focus here is on languages from the same family.)

Answer 1

One can take several approaches, which can also be combined.

A haphazard approach: Always carry a notebook (or a digital equivalent) with you in which you can make notes about things that create grammatical difficulties or raise grammatical questions. At regular intervals, look up these issues in a grammar and/or ask friends or colleagues what the correct expression is. You can then turn the correct expressions into cloze tests that you can add to your spaced repetition system.

A systematic approach: Work through a book with advanced grammar exercises, e.g. for German: Lehr- und Übungsbuch der deutschen Grammatik by Dreyer and Schmidt, or Übungsgrammatik für Fortgeschrittene by Scheiner and Hall. Check each answer in the answer key. Turn each exercise with an error into a flash card (as a cloze test). This is the most systematic approach, but requires a lot of discipline.

An input-based approach: While reading or listening (radio, video, conversations), take note of grammatical features, phrases and sentence constructions that are different from what you would expect or that you would not use spontaneously due to unfamiliarity with the required grammatical features. Look them up in a grammar book to check that you understand them correctly. Turn these phrases and sentences into flashcards. You can also add examples that you find in grammars or dictionaries.

An output-based approach: Write short text and find friends or colleagues who are willing to correct them. Look up any errors in a grammar to become aware of the relevant rule. Find additional example online, in grammars or in dictionaries. Turn these examples into flashcards.

"With a little bit of help from my friends": Tell friends, colleagues, etc. that you want to perfect you foreign language skills and that you would appreciate it if they could correct you (at least in specific context; this would not work all the time). Take note of these corrections (at least mentally, until you find a good moment to record them in a notebook, smartphone etc.). Then check grammars, dictionaries, etc. to find more examples of the same grammatical feature and turn them into flashcards.

Obviously, these approaches don't exclude each other.

Answer 2

Grab any (small) book you like and memorize every single sentence in it. Memorization is effective for the same reasons playing the same piece of music over and over again is effective for professional musicians.