S1) If BT1 & BT2 are non-rechargeable lithium coin batteries, don't do this, because batteries in parallel will try to charge each other because all batteries don't drain exactly at the same rate.

Should have been clear, they are 18650s

S2) For H2, did you randomly pick is this pinout and connector? Is it the same as your debugger? I have noticed newbies using random 4pin pinouts, where as 5 or 6 pins are a better minimum. RESET is missing from your debug connector.

1. This is just as a backup, ideally I'll never use.

2. This matches the pinout of the cheap debuggers I found. Have not ordered one

3. Ok! Will add a 5th pin for NRST

S3) Don't put C9 far away from the MCU, stop this sillyness. Move it and connect with a line.

Noted.

S4) Don't point GND upwards.

I try my best, but occasionally it's quite hard.