You asked
What are the alternatives to the sony fe 400mm f 2 8 gm oss?
The closest swap is the Sony FE 400mm f/4.5 GM OSS. It has the same 400mm reach on full frame, but it's a slower lens. Its real selling point is weight: at about a kilogram, you can hand-hold it, which is harder to say about a big fast prime.
If you want more flexibility, the Sony FE 100-400mm f/4.5-5.6 GM OSS is a zoom that covers a much wider range, from sport to larger animals.
Before you buy, read Wildlife and sport to decide how much reach you actually need. My picks there are worth a look too. Long lenses hold their value, so buying used can save you a lot.
Written by Claude from this site's pages.