Google said this was "5 days old", but obviously it lied and this is a year and a half old... But anyway, I was wondering if you had looked into raw CAN decoding and apparently you were thinking of it!
I think the OBDKey can be put into a mode where it offers raw CAN sniffing, but I've yet to see a Bluetooth OBD adapter that did this reliably.
An MCP25xx hat on a Raspberry Pi (SPI-to-CAN) had no trouble with my Subaru Outback's 500 kbit bus, however it seems to be running at lower bus load than the Bolt. Might handle 500k on the Bolt with no problem.
A Canable USB-to-CAN adapter works perfectly with the Bolt. Unfortunately Ethan doesn't seem to be making them any more as they've been sold out for a few months, and the isolated Pro has been sold out for nearly a year. Not sure what's readily available and cheap right now. There are quite a few alternative hardware designs using STM32 uCs that are supported by the candleLight firmware project.
At least some of the CAN data seems to not have changed since the Volt - for example speed in CAN ID 0x3e9 is confirmed to be present in a 2020 Bolt.
Supposedly there's a second high speed CAN bus accessible from the OBD connector, but I haven't found a decent OBD connector breakout to use with this and my DB9 adapter cable only supports the primary bus (I think, I need to see if maybe the other CAN pair is on another pair of DB9 pins...). Every one I've found so far was garbage.