Convert a hobby-level PCB into a PCB ready for mass-production

I have an existing PCB design for a handheld device and would like it redesigned for easier manufacturing and assembly. The current design uses: * Seeed Studio XIAO ESP32S3 Sense * 1.44" SPI TFT display module * MAX98357A amplifier breakout module * 3 tactile buttons * LiPo battery * Slide power switch * 8 Ω speaker The existing design functions correctly. My goal is to reduce assembly cost and make the design suitable for contract manufacturing (PCBWay assembly or similar). Required modifications: 1. Mount the XIAO ESP32S3 Sense directly to the PCB using castellated pads instead of through-hole headers. 2. Add a JST battery connector and integrate a PCB-mounted power switch. 3. Add a battery fuel gauge IC that reports state-of-charge over I2C. 4. Replace the MAX98357A breakout module with the amplifier circuitry directly on the PCB. 5. Add a JST speaker connector. 6. Review the entire design and recommend any additional DFM improvements that reduce assembly cost or improve manufacturability. Constraints: * Existing enclosure should require minimal modification. * Existing display module should continue to be used unless there is a strong reason to change it. * Device dimensions should remain approximately the same. * Design must be suitable for assembly by PCBWay or a similar contract manufacturer. Deliverables: * KiCad project files * Updated schematic * Updated PCB layout * Manufacturing-ready Gerbers * BOM with manufacturer part numbers * Pick-and-place/CPL files * Brief document describing design changes and manufacturing recommendations Please include examples of previous PCB designs you have completed, particularly products prepared for volume manufacturing. I have attached: * Current Gerber files * PCB images * Photos of assembled prototypes * Component links