We investigate AlN/GaN MISHEMTs fabricated on a 200 mm Si substrate with an in-situ SiN as the gate insulator and access region passivation layer. The impact of SiN thickness scaling on the large-signal performance is analysed using pulsed IV characterisation combined with first-order load-line-based formulae to quantify the degradation in Pout and PAE stemming from increased trapping-induced current collapse associated with thinned-down SiN thicknesses. At 28 GHz, large-signal characterisation, including tuning up to the third harmonic, reveals a competitive linear gain of 15 dB and PAE of 62.5\% at 6 V, making these devices suitable for user equipment intended for FR2/FR3 operation.