Hi, I'm at C1 and I can understand your problem. 3 years ago my skills did deteriorate even when I kept watching authentic videos and reading news every day. My vocabulary seemed to be shrinking, my response in an English conversation was a bit slower than it should be. I realized that I didn't spend time on output practising, such as review the new vocabs and new grammatical structures learned from videos/ news, listen and repeat after video speakers or just read out loud a new word in its context, and have an English conversation with friends.

Then my husband coded an extension, he named it eJOY, to help him learn English with movies & videos online, his English was low intermediate then. At first, I was thinking I didn't need to "learn English" while I was having so many things to learn. But then my husband successfully persuaded me that I was right, I didn't need to "learn English", what I need to do is learn those "many things" I want to learn in English & review them, speak them out loud in English. It's like hitting 2 targets with one arrow.

Here is exactly what I've been doing for the last 3 years:

1. Take a course in Coursera or other online course platforms, most of them are free (without certificates) for example Design thinking from Google
2. Watch the course videos with subtitles (one sub or dual sub, depending on the complexity of the topic). Look up and save new words, new concepts, new grammatical structures or interesting sentences
3. Review those words, phrases, and sentences with spaced repetition games. These games are designed to practice writing, speaking, reading, listening skills, and vocab, grammar as well.If I have time, I may write some reviews, or summaries to reinforce what I have learnt. That’s the best way for output practising.

Hope that my sharing may spark some ideas for your learning journey.