Sidebar, if i'm looking to darken the final color on the guitar, should I stain black first, sand back then dye red or should I just mix the dyes (black+red) and stain multiple layers?
You won't be able to mix those two products.
The Angelus Leather Dye is alcohol-based (and work really well btw) the Feast Watson is an oil-based stain.
Two different solvents and two different pigments used in each of them. They won't mix.

I have used the Angelus alcohol-based dyes and mixed them with powdered aniline dye which is first dissolved in alcohol with no problems. The Angelus dyes are essentially the same thing but in a liquid form instead of a powder.

This is my PRS-1TS which I used those dyes on:

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