I think your right given what I've read about the quality of some scopes. Fieldcraft it is then.I think you'd be better off spending the money on a quality pair of wildlife binoculars which will last many years - Zeiss and Leica are about the best of the rest after Swarovski. Then work on fieldcraft, observing [a] what you're trying to photograph and its habits and how to approach it or tuck yourself away to wait for it.
If 900mm on a crop isn’t enough you are doing something wrong! Have you found life on The moon to photograph!
That's an oversimplification. Sometimes it's distance/area v the size of the object being photographed. Particularly with birds.