PCB Design Review & Debug

I run a small electromechanical engineering firm and design my own control electronics. I've taken three board projects through two prototype spins each and hit walls I can't isolate on my own. I'm not looking for a redesign or finished files — I do my own design work. I need an experienced hardware engineer to review my schematics, tell me what's wrong, and tell me what to change. I implement from there. The three boards: 1. Speed control board (Gen 2) — drives an off-the-shelf motor speed controller with a variable speed signal. On the built boards, the variable speed isn't actuating at all. Also has a prox-driven error circuit. 2. Sensor interface board — 5V in, boosted to 12V, optocoupler returns the signal to keep my core system at 5V. Regulation/signal problems on the fabricated boards. 3. Main Arduino Mega control board — Mega 2560 Pro based; 5V→12V on-board conversion to power field boards, 5V distribution, optocouplers for signal conditioning, and a Cat6-style HMI connector on a hardware serial port. Where I keep missing: I'm strong on the firmware and system side but self-taught on PCB design, and my misses cluster in a few places — interface mismatches between my output stage and off-the-shelf parts (signal type/level, shared grounds), switching-regulator layout (feedback divider, loop area), and optocoupler circuits (LED resistor sizing, logic inversion). On the built boards some functions work and others are dead, and I haven't been able to fully isolate why. I need someone who can read a schematic and reason through that quickly. I work in EasyEDA and fab through JLCPCB. You don't need to own EasyEDA — I can export schematics and layouts as PDFs or standard formats. What I need is clear, specific direction on what to fix and how.