Here’s what I do, not sure if it will help you: I need checkmarks a lot - but I also need other symbols a lot - so I created a board just for the symbols that I re-use all the time. So when I need a symbol and I know it is in my collection, it’s easier for me to go to my board and copy it rather than try to use a built-in Miro feature. My “personal collection” board probably has well over 200 or even 300 different images, icons and shapes that I re-use all the time.
Especially with checkmarks: I have black-and-white ones, green ones, square ones . . .