As a general rule, you can merge layers in Photoshop without losing effects by flattening the image. Flattening combines all visible layers into a single layer, which is then placed on top of the Background layer.
Any hidden layers are preserved. If you have applied any filters to your layers, those filters will remain intact when you flatten the image.
To flatten an image, go to the Layers panel and click the Flatten Image button. You can also choose Layer > Flatten Image from the menu bar.
If you have any hidden layers, a dialog box will appear, asking if you want to discard those hidden layers. Click OK. That’s it!
Flattening an image is irreversible, so be sure to save a copy of your original image before flattening it.
Now that you know how to merge layers in Photoshop without losing effects, go ahead and experiment with different combinations of filters and adjustment layers. Remember to always save a copy of your original image before flattening it.