This work presents advancements in precision longitudinal beam diagnostics for Free Electron Lasers (FELs), integrating zero-bias Schottky-diode-based THz detectors and upgraded electro-optical bunch arrival-time monitors (EO-BAMs) for low-charge operation. The developed THz detector achieves ps-scale response times and 70 GHz intermediate-frequency bandwidth, enabling single-shot THz detection across kHz–MHz repetition rates. Characterized from DC to 5.56 THz$^{*}$, it can serve as a critical tool for bunch compression monitoring$^{**}$ and lays the groundwork for future development ultra-broadband THz spectrometers. Concurrently, a novel printed circuit board (PCB) pickup structure enhances EO-BAM performance, preliminary results gave a slew rate of 275.7 mV/ps $\pm$ 34.6 mV/ps for a peak-to-peak voltage of 4.16 V $\pm$ 0.31 V at 3.45 pC after de-embedding$^{***}$. Optimized PCB materials and planar designs improve signal integrity, achieving a simulated jitter-charge product of 9 fs·pC$^{****}$. This upgrade enables reliable operation at 1 pC for XFELs and ultrafast diffraction facilities while enhancing timing resolution in standard modes. The PCB architecture enables unprecedented flexibility for future multi-functional diagnostics. These innovations address critical challenges in low-charge, high-repetition-rate FEL diagnostics, advancing real-time beam characterization and accelerator optimization.