How to lock in new words?

Learning new vocabulary continues to be the hardest and most depressing part of my Japanese learning journey (after 5 years I’m somewhere between N4 and N3). Like literally soul crushing. My retention rate is barely above 50% and I only do 2 new cards per day and these are all words I encountered in real life. I don’t know what else to do.

• I use jpdb.io to learn words directly from the book I’m reading.
• I use my own mnemonic.
• I spend now maybe ~20 minutes per day doing flashcards. I can’t do more.

Is there a more gamified / interesting way of doing flashcards? I feel learning grammar is much easier. I’m in the 98th percentile for IQ and I’ve always done very well in programming/math but I feel like a total idiot when I’m studying Japanese and this is starting to have an impact on my wellbeing (though I absolutely don’t want to give up).