regarding the USB-C option, my experience is align with
I use the official Raspberry Pi4 USB-C powersupply with 15 W and measure with my low load profile - plex only and not converting - with a wattmeter a total load of
10 W when all disks (2 SSD, 2 HDD) running,
7 W on Pi 5 idle and disks sleeping and
up to 12 W on short peaks.
There are no indications on low power in dmesg.
With that the official Raspberry Pi5 USB-C powersupply with 27 W may be a choice to handle higher load profiles.
If the issue on 12V still persists even with a different manufacturer it seems that the hat’s power converter has an issue, maybe you can get in direct contact like here