I’m taking the following approach:
I have *** and ß both set (separately) to expand to ✱. Triple asterisk is efficient on macOS and triple vs double avoids issues conflicting with Markdown syntax. The ß symbol isn’t one I ever use in regular typing, but it’s a long press of the s key away. Long press and then slight slide to the right seems like an elegant option if I don’t want to hit Shift and the Symbols to get to asterisk on iOS. I chose ß because it doesn’t look like the letter of that key (vs. à, ò, etc.). Plus my left thumb easily long presses the s key.