I still don't get the meaning of those arrows.

What i think is the different types of arrows is different types of love. For example she pointed out on one chapter that there was a type of arrow for love at first site and it was a pointy arrow, I’m not sure what type of love sayo has though but I’m suspecting that it’s a type of love that’s been with her for a long time (just like the mc girls mother has) and I’m also sure of that arrows can change shapes.
When Sayos arrow began to become messy and bumpy, I think it meant that she was having complications towards the feelings for them.

Other wise this is just how I think it is through what I’ve read