While the dictionaries say すぐ is often written in kana, in real life, I come across this in it’s kanji form quite often. Mostly in emails, but also Line messages. The Wanikani kanji 直 doesn’t list the すぐ Kuniomi, nor, the “soon” meaning. I think this is one that would be useful added as a vocabulary word.
I think WK wants us to focus primarily on the “fix” meaning and readings as they are more common, but I agree.
I’d like to see す (ぐ) added to the kunyomi readings on the kanji page at least, whether or not a vocabulary item is added for 直ぐ.