Q.tree lilies
Should tree lily bulbs be split after a few years?

A general rule is if the plant is crowded and shows signs of less flowers or stunted growth, it's time to split your bulbs.
The best time to divide can vary, Fall is a good time, and gives the bulbs time to develop good roots.
Spring division can be done, but you may have less flowers until the following year.
https://www.gardeningknowhow.com/ornamental/bulbs/lily/growing-tree-lily-bulbs.htm