Talking about your leeches is the best way to learn them anyway.
For the meaning, I usually look at the kanji and see the 金 part that kind of always resembles a metal of some sort, and the right side kind of looks like a pencil. So the metal in a pencil would be the “lead” (I know it’s carbon but w/e.
For the reading, we already have the lead, so now we need what’s going around the lead. The best thing would be some raw reeds, since they’re often hollow. So we need some 生(なま)り.
In desperation I’ve taken to writing such leeches on a post it note and sticking them on the toilet door. This seems to be working. Very strange how some become leeches and some do not. There seems to be no rhyme or reason. I can remember lead easily (sorry) but can I remember 混じる? No I can not. My brain won’t have it. It literally rejects it.
This connection to lead seems to be present in many languages. In Polish it also has the name related to lead. “lead” - “ołów”, “pencil” - “ołówek”.
If I’m not mistaken, before modern pencils, often lead styli in wooden “frame” were used as drawing tools ( silver-point). Also, when graphite has been discovered in 1564, it was initially thought to be some kind of lead ore (not carbon). That’s why the “core” of the pencil is still called “lead” according to wiki