The current mental image for 丸, using the nine and drop radicals, is ‘What’s something with nine sides? Just add a drop and you’ll have something that’s almost a circle.’ Now I know it’s hard to come up with memorable images for thousands of entries, but that one’s pushing it for me.
I find I can remember it better as ‘You take the number 9 and you drop it, and oh no, the drop broke off! What are you left with? A circle.’
For some reason “Maru the cat” was enough for me to learn this one, despite not knowing anything about Maru on youtube. Funnily enough, you can actually see a cat in the Kanji: you can see his left side, he is stretching (the tail on the right, his butt high up in the air, his front paws on the ground in the bottom left).
Then I associate Maru with a cat rolled up in a circle as the reading mnemonics suggests and that’s how I memorized it. The meaning mnemonics for 丸 is indeed quite convoluted.
Speaking of mnemonics suggestions, there’s also a very effective reading mnemonic for the kanji 引 (pull) that I feel like Wanikani missed out: ひ, like the sound you make in the effort to pull the string of a bow. I find it to be more immediate than the “heat” association
i didn’t used this mnemonic at all. i learned the adjective 丸い (まろい) “round or circular” previously while studying grammar and that’s how i know this kanji. maybe it works for you too.
It can be explained very simply
Let’s say lim n → ∞ = ∞ via:
Which is to say that as our n number increases, we are getting closer and closer to a circle. (We will treat a circle as a ∞-gon/polygon with infinite number of sides)
Now imagine a 9-gon/nonagon which already resembles a circle enough as it is, and imagine the drop as being an extra side which will make it a 10-gon and it’ll look even more like a circle than before! This is a very simple explanation for this kanji.
Nine drops is not a round number. Add one drop and you’ll get a round number.
Well, perhaps more to the point, if you add one to 9, you get 10, and 0 is a circle. 
Uh… ish.
I work in IT, and see the word 丸め (rounding) quite regularly, so it was more natural for me to abstract from the circle. And the nature of the 丸 itself is a concept of regularity and enclosedness, so not focusing on the shape could be beneficial.
I used Youtube Maru to remember this reading, since he is quite a unit (big and round), and his face is round:
