Your only cost-effective lens which is longer than 300mm is a teleconverter for your 75-300. 1.4x TC will probably still allow autofocus with your lens and body but I am not entirely sure (the combo will have F8 speed and T7 can be limited by how slow the lens can be for AF to function).

Second best option is cropping your images. There is no reason why you should not do it except some quality loss.

There are a lot of older used AF lenses without stabilizer which are not terribly expensive but they can only be heavier. Another option is a 500mm F8 reflex lens which won't have AF.

Your only cost-effective lens which is longer than 300mm is a teleconverter for your 75-300. 1.4x TC will probably still allow autofocus with your lens and body but I am not entirely sure (the combo will have F8 speed and T7 can be limited by how slow the lens can be for AF to function). This will also make viewfinder darker because speed is lower with TC.

Second best option is cropping your images. There is no reason why you should not do it except some quality loss.

There are a lot of older used AF lenses without stabilizer which are not terribly expensive but they can only be heavier. Another option is a 500mm F8 reflex lens which won't have AF.