Check with powertop that runtime power manage is enabled for devices (tunables are “Good”).
It looks like it has a RTL8111H for Ethernet, which is known to be problematic with sleep. My machines don’t go below C3 due runtime power management being disabled for Ethernet, but enabling it causes it to fail to come out of suspend correctly.
All UEFI system firmware uses edk2. It’s not just the reference implementation; Pragmatically, it’s the only implementation. Independent BIOS vendors (IBVs) like AMI, Phoenix, and Insyde have built all their tooling for and around edk2. Companies like System76 and Purism use it as a UEFI payload for their coreboot based firmware.
coreboot isn’t a UEFI implementation. It is comparable to the UEFI SEC+PEI phases. It then hands off control to a payload. If you want UEFI, that’s going to still be edk2.
What firmware version are you on?
Did it boot to firmware-update and flash the new firmware when you reboot?
There is a CLI tool that you can use to schedule it: https://support.system76.com/articles/system-firmware/#starting-the-update-cli