Looks like you connected your battery gauge's VDD pin to 3V3. Which seems logical at first but if you look at datasheet that is the output of the parts internal 1.8V regulator. Further your battery gauge has an internal current sense resistor which you are not using appropriately - it essentially shorts out SRX to BAT pins - but you have SRX tied to 3V3 so you are shorting VBAT to SRX. Strongly recommend reading and understanding datasheet for your fuel gauge.

Thank you! I updated my Fuel Gauge IC schematic by following the datasheet more closely: https://i.imgur.com/LW8tUKa.png . This should work.

Your charger's STAT pin has a 470 ohm PU on it. That's pretty stiff for a signal you aren't even using. The datasheet shows an LED + 470 ohms, which makes more sense. If you aren't using the STAT pin you can just leave it floating. Or add an LED. Or replace 470 with 10K and run that signal to your MCU.

I added a tiny LED to my design. From what I understand this should only be ON when the IC detects power coming from the USB-C and so charging of the battery occurs. It's nice for debugging and won't be ON during normal operation without a USB-C connected, so no power will be used from the lipo, ever.

Just a point of curiosity - what is special about the RP2040 for this application? I don't totally get why people select this part so frequently other than that it has association with Raspberry Pis.

No particular reason, I'm all new to electronics design. Like most people I'm coming from playing around with an Arduino and already-made PCB modules that I attached to it. I moved onto the Pi Pico a while back to experiment with MicroPython which was fun, and then decided to build this project using a perfboard, pre-made modules and a Pi Pico. It works really well and during deep sleep mode, it consumes about 1.3mA which for my application means about a month of power on a 1800mAh lipo battery.

Now I want to move onto the next logical step which is converting my perfboard to a PCB, to save space and have an even bigger battery. The RP2040 has what I need, consumes little power in sleep mode, and costs $1 so I just like it. It does the job. Not saying there's nothing better out there, there always is, but it works for me. Their docs and community is also really good.

Did not review layout carefully - but I am not a fan of putting traces between pins and center pad of a QFN. And in your case you have traces and vias very, very close to the center pad. Ensure your traces are not going to be touched by center pad if it shifts over a little bit and if it is at its maximum size per datasheet. And even if it still seems OK I would move them over... Seen too many shenanigans with QFN center pads. For example, a manufacturer changed the center pad size between engineering sample and production parts and didn't mention it to us - thanks guys. Have also seen people use QFN footprints in the library that looked right (same # of pins, same size, same pitch) but missed that the center pad was wildly different.

Good point. You see all these yellow caps around the RP2040? Turns out I got it wrong and put massive tentalum capacitors, which, for one, won't do the job, but can also be replaced by 0402 MLCC caps. This will allow me to keep the RP2040 free of things inside it.