Midwest US originally:

Thingy if it's attached to me. Thingie if it's a cute small object I enjoy the appearance of.

ex:
"this is my thingy"
"Ooh what's that thingie?"

Im from EU and I think it’s an adorabelization so it’s thingY

Thingy - Western US

Thingy. Thingie would be pronounced like Thin-jee

"Thingy" here in southeast Pennsylvania, U.S.A.

I've never written it.

Edit to add that if I did it would probably be with an ie as a y would imply more of a j sound.