When displaying a schematic in an area with complete different/untouched terrain or on top of already existing buildings I cant navigate/see what I have to remove.

Please support an transparency setting for schematic placements.

Translucent schematic rendering is already possible, for that in Visuals enable the renderBlocksAsTranslucent option and set the alpha value in ghostBlockAlpha.

However if you mean just for clearing out areas before starting to build, then I would recommend instead that you disable both the schematic block rendering and the missing block overlay types, that way you only get the red or purple overlay around blocks that you have to mine away. That's by far the best option for clearing out areas before starting to build. So in Visuals disable enableSchematicBlocksRendering and schematicOverlayTypeMissing.

I've tried flipping the renderBlocksAsTranslucent option on and off and the blocks are always shown as non-translucent, would you know why? I'm on 1.17 with the 1.17 litematica and fabric

The translucent rendering simply does not work in the current 1.17 version, due to the vanilla rendering changes in 1.17 making the old approach now non-functional.

I'll look into fixing it at some point, but I'm not actively working on the 1.17 versions currently, as I want to rather work on the mod code rewrites and clean-up that is happening in 1.12.2. There are lots of various other issues that will benefit from those rewrites or bugs that are only properly fixable by rewriting some things.