For example, lately, I came across the Latin expression 'bona fide'. I don't remember I've seen it before (so, I suppose, it's used relatively rarely) but if I do encounter it in speech or writing, I want to know what it means. All those word cards, notes — all that is crap, based on my experience, it's not working. I incorporate a new word into my vocabulary (my brain does) when I either see it often (at least, from time to time) or if I use it myself. You can't do the latter because it's too show-offy (it seems so to me, as most Latin phrases are). How can I learn it?
Two of the options you listed are pretty decent:
Spaced repetition. You said "cards", so you might just be using regular flash cards, but using spaced repetition software can really help with long-term recall. I use Anki, but you can use whatever software you like.
Spaced repetition is not the best way to learn something in the first place, though. It's there to help you remember it once you've learned it. Ideally, you should add cards for things that you've learned well, but are tough to remember because you don't see them very often – your exact use case.
Using them yourself. You've ruled this out as "too show-offy", and I can understand that, but if a word is appropriate in context there's nothing wrong with using it.
What if it's not appropriate in context? Well, a lot of people like words. You can always create the appropriate context by talking about the word. If they don't know the word, you can teach it to them. My friends and I teach each other words all the time, and it's pretty fun. We often make a game out of it by looking for every opportunity to use those new words with each other, even if it's a bit of a stretch sometimes.
Honestly, it can be pretty funny if you use a new word just a bit inappropriately, and having fun like that can actually make it easier to remember new vocabulary in the long term. Personally, I like to have fun with language. When you get excited about something, or engaged with something, you tend to remember it better than if it's boring and you're just slogging through it.
Taking notes can help too. I don't refer back to my notes very often, but I do like to take them. Just the act of writing something down (especially if you do it by hand) helps you remember, even if you never look at your notes down the road. I know you said it's not working well enough for you, but I wouldn't necessarily give up on it entirely. It's not enough on its own, but it can help you build a foundation in your memory, which you can build on by actually using the word.
Anyway, that's all I've got for you. I know it's not much new, but I feel like you're already aware of the best options and you're ruling them out. I hope you can find something that works for you.
Incidentally, bona fide isn't all that uncommon. In fact, I used it in a text just last week (I remember, because my autocorrect always wants to change "bona fide" to "bonus find". How annoying.) You can find plenty of examples in news articles and headlines.
Which brings me to another suggestion: you could look up these words using Google, click on the option for News, and then start reading articles where these words or phrases are used. Perhaps reading a phrase in context would help you better remember what it means.
For example, reading usages such as:
- Alton Mason Is On His Way To Bona Fide Superstardom
- Become a bona fide data professional with this SQL training
- For the next year, I braved bona fide tree ornaments and bona fide wrapping paper.
- ...a sophomore season that has turned him into a bona fide star.
might help you create mental pictures that would work better than rote memorization.
All those word cards, notes — all that is crap, based on my experience, it's not working.
Just because something isn't working now doesn't mean it won't eventually work. Give it time. Most non-native speakers continue to learn throughout their life. I don't suppose you are in a rush to learning difficult words or expressions, right?
You can't do the latter because it's too show-offy ...
So what if it is or if it seems that way? If your primary concern is to learn a particular word/expression, then don't let that feeling deter you from using it. At worst, you will be corrected if you use it incorrectly. Your close one's won't mind. And the rest might frown if you misuse it. But that shouldn't stop you from doing the right thing.
Try to connect the words or expressions with real people you know. That is one of the several types of Mnemonics. Use the word in a sentence that has something to do with your friend, family, colleague, crush, or enemy. Funny things (e.g., rhythms, songs, jingles, phrases) have a way of staying in our memory for a long time. Make the sentence funny or lame, it will work.
Make the word/expression personal.
"Their [Mnemonic's] use is based on the observation that the human mind more easily remembers spatial, personal, surprising, physical, sexual, humorous, or otherwise "relatable" information, rather than more abstract or impersonal forms of information."
It's not clear why you'd want to actively learn new vocabulary items (as you would a history lesson from a textbook), or what difficulties you're facing exactly. Learning vocabulary by reviewing a bunch of cards or whatever isn't something that's natural: as a native speaker of a language, I've never reviewed or used flashcards to learn its vocabulary, and I bet you haven't done anything similar either. And I've never had any real issues expressing myself in my first language.
You learn words by seeing or hearing them used in different contexts, and then intuitively (or with some help, say an online dictionary) absorbing them.
Whenever you find yourself actively learning a word, it means you haven't seen it in its natural habitat sufficiently often. Look up its meaning and move on. The more often you encounter the word, the faster your learning process will go. (You'll occasionally come across words whose meaning stubbornly eludes you, and you'll have to look them up every damn time – in those cases people naturally resort to mnemonic techniques.)
You're obviously not a complete beginner, so you don't see English as something so foreign you have to write it down and review 10 times as though it were random facts about Roosevelt. It should all be smooth sailing from here on out.