Some other things to try, since we had a similar issue when we first switched to Unity.
In your atlas texture settings you have "Non-Power of 2" set to "ToNearest", but in your Spine Texture Packer settings you don't have "Power of Two" toggled. Unity will stretch your texture to be the nearest power of two with this setting active. Setting this to "None" will help fix any quality loss caused by Unity stretching your texture to be larger than it actually is. Alternatively you can choose to toggle the "Power of Two" option in the Spine Texture Packer settings, and leave the Unity "Non-Power of 2" as-is.
Next thing to note is your Max Size for your texture. If you packed an 8k texture with Spine, but then limit it to only be 4k in Unity, you'll see a loss of quality. Try increasing the Max Size and seeing if the preview shows the texture having a larger size afterwards. Always make sure that this Max Size value is larger than or equal to your actual texture size to avoid Unity downscaling it automatically for you.
I recommend using Point filtering, because chances are your Spine symbols are already anti-aliased, and will not benefit from additional hardware anti-aliasing.
If you've done all this and it's still blurry, then I think there must be something wrong with your Texture Packer settings in Spine. Perhaps your individual symbols cannot fit into the size you've limited your texture atlases to (e.g. your "Tree" symbol might be 5000 px tall, so it must be downscaled to fit into your 4k texture atlas, though if that were the case you should be seeing multiple atlas pages, and not a single page).
