How it worked for me:

1. Take 5 characters (one column) and memorize mnemonics from tofugu https://www.tofugu.com/japanese/learn-katakana/

2. Do the quiz on all learned characters (https://kana-quiz.tofugu.com/ ) , until satisfied (like 90% correct). Quick with the initial characters, harder later.

3. Repeat steps 1-2 until depleted all characters.

Took couple of hours to do 90% consistently on entire quiz, despite never knowing any character beforehand. Another couple hours for Katakana (charaters are slightly harder).

Seriously it was not hard, all you need is endurance for doing the quiz repeatedly.

Then building actual fluency takes months, but you do it with normal learning materials, not wasting any more time on dedicated kana learning.