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.’
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.
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.
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.