I would NOT consider myself a serious wildlife photographer with a crop sensor.
Full frame. Mirrorless.
And let me also suggest that Sony’s A7 IV is the superior camera out there. Sublime colors and fast enough frames per second. 61 megapixels so you can crop yo your heart’s content.
Sony manufactures the sensors for their own products as well as Fujifilm’s and I have the GFX100 and the 50R and I could not possibly be happier.
Sony A7 IV.