If you're worried about mixing wood tone colours: Liquitex has a few very nice wood tones, straight out of the tube or rattle can.

Trying to understand the process you have in mind. Why did you sand it so much, damaging your artwork to such extend? It seemed a valuable touch-up candidate from what I see on the photos.

I don't know about Lectronamo, but my Sterns (QS and SG) don't have connectors between the head and cabinet wiring, like Williams and Bally have. Makes it less convenient to take off the head for transport. I plan on adding connectors when restoring my games.

Wicked Colours are not opaque enough for this purpose.
I'm using Opaque series from Createx as much as possible as the basis for my colour mixes.
Or if you prefer: Createx also provides Autoborne Sealers in many colours to be used as an initial primer.

Obviously it's your decision, but I would keep the wood tones as such. You might regret later on that you painted them black. A thin, transparent layer of wood tone is what I would do, just to refresh the areas, whilst making sure to keep the wood-structure visible. But then again: it's your game, you decide

Looks great!

Screen printing is the route I'm considering as well for fine details (text for instance, or dithered gradients). After all it is the technique originally used to print a playfield. I've done it a lot, but it's a long time ago. Actually screen printing in itself is not so difficult. But it requires quite a setup, and many steps, before your art is on the screen, ready to be printed.

The result will be stronger than waterslide decals and more precise than stencils made with a cutter. But for sure it will not be a timesaver.