Like most of the other responses, I don't think there's a problem with your lens. It seems fine and I really don't see where there would be much room for improvement.
I would recommend you shoot some tests with your 150-600mm to find it's optimal apertures and focal lengths at various distances. You probably would see some improvements stopping down from wide open, although that will require using a higher ISO. The 1/320 shutter speed you used for this shot is just about the bare minimum for quick moving wildlife. If the bird had been on the wing instead of "posed", you'd have needed a faster shutter speed.... probably 1/640 at a minimum.
Another thing... If you have a "protection" filter on the lens, try some shots without it. Might surprise you!
Many have mentioned the lack of a catchlight in the bird's eye.... and I agree. That's important to make animals appear alive. Without a catchlight, they look dead. For several reasons, I suspect your subject was in shade when this shot was taken... which might explain the lack of a catchlight. One possible solution is to use weak fill flash. When using a telephoto from some distance, that also often means using a flash extender. Here's a discussion and some examples of how that's often done: https://luminous-landscape.com/better-beamer/
In fact, I think that most of the "issues" with your images can be solved in post-processing and, if anything, that's where I'd suggest you spend some money, learn some tricks and work to improve the image. (Better editing software? Books or classes to learn to use it well?)
I used Photoshop... first simply "opening up" the middle tonalities of the image slightly using Levels (Do you use a calibrated monitor? If not, that might be necessary for really accurate exposure adjustments and may be a better place to spend some money, than on a lens). Then I used a "pencil" tool to add a catchlight to the bird's eye... That took several tries with different sizes and locations before I got something that I felt looked "right". I also "dialed back" the catchlight slightly... It's not pure white in the edited example below. Also notice how the tonal adjustments made the white ring around the bird's eye a little brighter, as well as increasing contrast slightly over the rest of it's feathers.
Because it appeared to be shot in shade, I also used Photoshop to add a weak "81A warming" filter to the overall image. Shaded subjects tend to be bluish, especially on days with a bright, clear blue sky. I was careful not to overdo this because of the bird's coloration.
The next thing I did was select the background (using the "magic wand" tool). I added slight blur to that.
Then I inverted the selection and did some SLIGHT selective sharpening of the bird and the barbed wire fence. Personally I think adding some blur to out of focus areas like the background here can make the in-focus areas appear sharper, without over-doing it. I also used the sharpening tool and blur tools in Photoshop to very selectively retouch some of the bird's feathers here and there... increasing sharpness in some of the softest areas with the one tool, while toning down some of the sharper areas slightly with the other.
It appeared to me that some sharpening had already been applied overall to the image... that was causing some excessive sharpness of some of the feather detail, "halos" and JPEG artifacts along some of the higher contrast edges, and seemed to be adding some granularity to the background. With only the JPEG to work with, I used a clone tool to eliminate some of the halo/artifact problems along various edges (under the bird's beak, under the barbed wire). If I'd been working with a RAW original instead, I might have been able to avoid both those artifacts and the background granularity by selective sharpening.
Overall, I only spend a few minutes retouching, tried to keep it to a minimum and not to overdo it. (I often see what I consider over-sharpened images being displayed online... That makes my eyes bleed!
Depending upon what was going to be done with the image... how large and how high resolution it was going to be displayed... I might do a little more work on it. For example, the top of the bird's head is still a bit "soft", when it's viewed in larger sizes. But, overall, notice the slight increase in "pop".
Here's the result and your original to compare...