Gold nanoisland sensors are commonly optimized by following the largest optical response, although a single maximum does not define the best film for every readout. This paper determines how nominal Au thickness separates the functional windows of a graded, annealed gold nanoisland film used for Raman enhancement, ethanol transmission sensing, water-induced ellipsometric contrast, and ellipsometric detection of chemisorbed 4-mercaptobenzoic acid. The common coordinate is deposited Au thickness on fused silica. The decisive morphological transition occurs between 2.0 and 4.4 nm: particle density decreases from 2090 to 88 particles μm−2, while covered area decreases from 0.49 to 0.21. These numbers distinguish a junction-dense low-thickness state, an open-contact intermediate state, an oscillator-dominant high-thickness state, and a continuity-threshold state. 4-MBA Raman detection is strongest near 1.6 nm, where the 1074 and 1586 cm−1 bands are most intense under both 532 and 633 nm excitation. Ethanol sensing is strongest around 3.2–3.5 nm, and water-induced ellipsometric contrast occupies the broader 2.0–4.0 nm interval. Bound 4-MBA ellipsometry shifts to 6.8–7.0 nm, where the far-field response is stronger. The practical answer is therefore readout-specific: 1.4–2.2 nm for Raman, 2.8–3.6 nm for volatile or condensable analytes, 6.5–7.1 nm for bound-layer ellipsometry, and 2.0–3.5 nm as the best multimodal compromise.