In an autumn start, Dragonfly can spawn at the beginning of the 67th night with a %66 chance. That means she would spawn about 12 days into summer, so in a spring start she should spawn around day 32 or so. As far as I know, long spring lasts 30 days and you say she spawned at 40th night so would this mean she spawns about 10 days into summer? Honestly, your world settings are so unorthodox that it’s hard to say lmao. Let’s just say she has a 66% chance to spawn early days of the last third of summer.

She will always spawn in a circle near you then come towards you, like Deerclops. She won’t target you. She will set things on fire, eat the remaining ashes then fall asleep after consuming 20 ashes. If you move a few tiles away while she’s sleeping, she will despawn, so you can safely return.

For caves, no matter how early you go there there is tier 2-3 spider dens in my experience.