We present comprehensive first-principles calculations of the electronic structure of van der Waals bilayers formed by bismuth chalcogenhalide monolayers, with chemical formula BiXY, a family of ternary compounds with X = S, Se, Te and Y = Cl, Br, I. Using density functional theory, we investigate 513 non-equivalent bilayer configurations, generated from 729 nominal stacking labels spanning different chemical compositions, in-plane registries, and layer orientations. Our analysis reveals a wide band-gap tunability, ranging from finite-gap semiconducting to near-gapless or band-overlap regimes, directly linked to the chemical composition and structural configuration of the bilayers. The study is motivated by the giant Rashba effect experimentally observed in BiTeBr and BiTeI, and aims at extending this concept to the broader BiXY family. We find that the delicate balance between van der Waals forces, governing the interlayer registry and hybridization, and spin–orbit coupling, strongly influencing the band dispersions, leads to fully tunable spin-texture patterns and highly adjustable Rashba splittings. In particular, the Rashba parameters and spin orientations are shown to depend sensitively on both stacking symmetry and halogen substitution, offering a high level of control over spin–momentum locking. These results provide a detailed microscopic understanding of the structural and electronic degrees of freedom that determine the properties of BiXY bilayers, and point to their potential as a versatile materials platform for spintronics and spin–orbitronics applications.