The basic question is: what will trigger the Wow!-factor for you
Based on the current equipment D5000 (12.2MP) with 55-300mm lens, I would consider some possible upgrade paths which may bring you closer to a Wow! factor.
In general, having more recent equipment will give you some benefits, considering that:
- a more recent camerabody will:
. offer you more (typically +-24MP for most recent camerabodies) MegaPixel, allowing to crop the picture in post processing (i.e. give you a virtual zoom factor of 1.4 (=square root of 2))
. have a more recent processor and thereby allow for higher ISO values (i.e. lower light) with the same level of noise.
- most recent lenses have an improved VR/VC, allowing for still sharp pictures at lower light or shorter shutter speeds
Still, the first equipment to get is a tripod, if you are currently shooting handheld, especially if you will investigate into heavier lenses.
Considering that the D5000 weights 588g and the 55-300mm lens some extra 550g, your current total weight is 1.138g.
A Tamrom 150-600 (the same goes approx for Sigma) weights 1.951g on its own - adding the 588g of the camera this gives you a total weight of 2.539g, i.e. slightly more than double of the weight that you currently hold.
At 600mm the field of view will be some 2°40’ on a DX body, compared to 5°20’ of the 300mm - therefore the « magnification » between the two will be a factor of 2,16.
(Please ignore the point of comparing the point of equivalent focal lengths between DX and FX bodies that Peter made for the 400mm, unless you have also a camerabody with an FX sensor)
Starting from here you have two choices:
1/ the weight factor is no issue:
The upgrade path could be to
- get a 150-600mm lens
- get along the tripod a Gimbal had, which is very convenient for heavy lenses
- get, at a convenient moment, a recent camerabody
2/ the weight factor is an issue
- get a smaller zoom lens (possibly the Sigma 100-400 suggested by Peter)
- get, at a convenient moment, a recent camerybody
Alternately, if you want to get into the AF-P lens set (which is not compatible with the D5000, but offer better prices):
- get a D5600 with a 70-300 AF-P lens - possibly in a bundle