For your situation, I would not try to memorize every useful word or idiom deliberately. That easily becomes boring and takes too much time. I would use a small “notice → recall → use → meet again” system.

A practical method without Anki

The most important point is that retrieval is better than rereading. Looking at a vocabulary list ten times can create familiarity, but trying to produce the word yourself strengthens your ability to actually use it.

For you, I would keep the amount quite small: perhaps 3–5 genuinely useful items a week, not 20 or 30. And I would include patterns and chunks, not only individual words:

• I can help with whatever you need.
• Your best bet is to…
• I have a few errands to run.
• It turned out that…
• I ended up…

These are especially valuable for speaking because you can retrieve the whole pattern rather than constructing the sentence word by word.

And don't worry too much if something seems to disappear after two or three months. Vocabulary knowledge is not simply remembered / forgotten. A word may move into weaker or passive knowledge, and the next encounter can strengthen it again. With extensive reading, TV, writing, and our conversations, useful vocabulary should naturally keep resurfacing.

For your B2→C1 goal, I would use this rule:

Don't ask, “Can I remember this word forever?” Ask, “Can I make this expression appear several times in my real English?”

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Input → notice useful chunks → retell → paraphrase → reuse in your own speech.
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Fluency development means building multiple automatic English routes from meaning to expression.

One idea → three routes

Once or twice during a speaking/writing session, take one useful thought and say it naturally in 2–3 different ways.

If it is difficult ->

a hybrid method: you try first, but only once. Don’t force yourself to invent three versions if that feels unnatural.

At your level, the exercise should work like this:

1. Say or write your normal version.
2. Try to express the same idea in one different way.
3. Then ask me for 2–3 other natural versions.
4. Compare them and choose one expression that feels useful enough to remember and reuse.

For example:

Your first version:

The job is sometimes boring because I do the same things every day.

Your own alternative might simply be:

Sometimes my job is boring because it is repetitive.

That is already enough productive work from you. Then I could give you:

The job can get a little repetitive.
Sometimes I get tired of doing more or less the same thing every day.
What I find difficult is the repetitive nature of the work.

The important part is that you make at least one retrieval attempt before seeing my versions. If I immediately give you three alternatives, the exercise becomes mainly receptive: you read them and think, Yes, I understand these. That's useful, but it trains speaking less.

And if you genuinely cannot think of another version, don't struggle for five minutes. Give it perhaps 10–20 seconds, then tell me:

“I can't think of another natural way to say this. Give me three alternatives.”

That difficulty itself tells us something useful: the idea currently has only one readily accessible English route in your mind.

For you, I would therefore modify my original suggestion to:

One thought → my version → one attempted alternative → ChatGPT alternatives → choose one.

I think this is much more realistic for B2→C1 than expecting you to spontaneously produce three sophisticated paraphrases yourself.

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When I arrived in Buenos Aires in the beginning of 2010, I could barely order food in a local restaurant. Two years later, I calmly explained the mechanics of Russian grammar to a Guatemalan friend… in her native Spanish.

1. Conversation, Conversation, Conversation

If there’s a “secret” or “hack” to learning a foreign language, it’s this: hours and hours of awkward and strenuous conversation with people better than you in that language.¹ An hour of conversation (with corrections and a dictionary for reference) is as good as five hours in a classroom and 10 hours with a language course by yourself.

There are a few reasons for this. The first is motivation.² I don’t care how cool your study guide is, you’re going to be far more invested and motivated to communicate with a live person in front of you than a book or audio program on your computer.

The second reason is that language is something that needs to be processed, not memorized. I’m no linguistics professor, but in my experience, staring and memorizing a word in a book or with flashcards 100 times simply does not stick the same way as being forced to use a word in conversation a mere two or three times.³

I believe the reason is that our minds place more priority on memories which involve actual human and social experiences, memories which have emotions tied to them.

So, for instance, if I look up the verb for “to complain” and use it in a sentence with a new friend, chances are I’m always going to associate that word with that specific interaction and conversation I was having with her. Whereas I can blow by that same word 20 times with flashcards, and even though I may get it right, I haven’t actually practiced implementing it. It means nothing to me, so it is less likely to stick with me.
(25 Tips to Learn a Foreign Language)