Affordable and decent bird photography setup? (600mm equiv)

I have a Fujifilm camera and decent lens that can do 600mm FF equiv is not cheap. I’m wondering what is the best option for bird photography at that range? There are some tiny 1/2.3 compact cameras like the Sony hx99 that doesn’t seem too good. There also seem to be an option of picking up a m43 camera with a 300mm (600mm...