It’s been 6 months and I’m still at level 30. I haven’t been able to advance a single level, because:
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whenever an older item (Guru / Enlightened) comes up for review, I remember absolutely nothing about it and so it fails several times down to Apprentice, at which point I have to slowly try to get it back into memory;
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during review I investigate all the “burned” kanji and radicals that I forgot and have to Resurrect them all
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this makes my review pile to be constantly growing, instead of shrinking, up to hundreds and now thousands
Therefore I decided to stop doing newer lessons until I see a zero in my review stack (which won’t happen anytime soon.) Fortunately I originally bought a lifetime account, so this is not costing me money.
But that still wasn’t enough, so I decided to follow this advice: How to kill a giant review backlog
I put my WK in vacation mode and started doing custom studies from level 1, one level at a time, for as long as it will take.
At the same time, I’m making my own Anki deck to practice WRITING all the kanji, without using mnemonics, because I understood that my trouble with similar kanji stems from the fact that I haven’t been writing them at all since I left language school.
So for every kanji, from level 1 onwards, I find the simplest or most common word whose reading is unique in the dictionary and I use that as my Anki question.
For example, this question:

Only has one possible answer:

(I know it’s 花火, but this is the card for 花 so everything else gets the hiragana treatment.)
I chose はなび because はな can be 鼻, 華, 洟… What I do is write 花 with my finger on my phone’s screen (it’s an AnkiDroid feature called whiteboard) then I flip the card and rate myself.
It’s still early to say if and how much it will help, but I’m seeing some progress.
I will share the deck when I get back to level 30, in case others want to use it.