noise617 That's what I meant, dmesg and journald already log persistently accross reboots, so if you try to boot with the bugged kernel, and then with the previous one, you should get usable logs.
sudo journalctl | grep -E "(xhci|\: usb)"
should give you errors related specifically with xhci (the usb controller) and usb devices.
I had some free time so I tried to compare boot logs with the bugged kernel, and with the last working one, and the issue seems to be situated here:
oct. 30 11:20:08 solus kernel: xhci_hcd 0000:00:14.0: Timeout while waiting for setup device command
oct. 30 11:20:14 tenroc-laptop kernel: xhci_hcd 0000:00:14.0: Timeout while waiting for setup device command
oct. 30 11:20:14 tenroc-laptop kernel: usb 2-3: device not accepting address 4, error -62
oct. 30 11:20:15 tenroc-laptop kernel: usbcore: registered new device driver apple-mfi-fastcharge
oct. 30 11:20:15 tenroc-laptop kernel: usbcore: registered new interface driver bcm5974
oct. 30 11:20:19 tenroc-laptop kernel: xhci_hcd 0000:00:14.0: Timeout while waiting for setup device command
oct. 30 11:20:25 tenroc-laptop kernel: xhci_hcd 0000:00:14.0: xHCI host not responding to stop endpoint command.
oct. 30 11:20:25 tenroc-laptop kernel: xhci_hcd 0000:00:14.0: xHCI host controller not responding, assume dead
oct. 30 11:20:25 tenroc-laptop kernel: xhci_hcd 0000:00:14.0: HC died; cleaning up
oct. 30 11:20:25 tenroc-laptop kernel: xhci_hcd 0000:00:14.0: Timeout while waiting for setup device command
oct. 30 11:20:25 tenroc-laptop kernel: usb 1-3: USB disconnect, device number 2
oct. 30 11:20:25 tenroc-laptop kernel: usb 2-3: device not accepting address 5, error -62
oct. 30 11:20:25 tenroc-laptop kernel: usb usb2-port3: unable to enumerate USB device
oct. 30 11:20:25 tenroc-laptop kernel: usb 1-3-port2: couldn't allocate usb_device
oct. 30 11:20:25 tenroc-laptop kernel: usb 1-3.1: USB disconnect, device number 4
oct. 30 11:20:25 tenroc-laptop kernel: usb 1-5: USB disconnect, device number 3
oct. 30 11:20:25 tenroc-laptop kernel: xhci_hcd 0000:00:14.0: dma_pool_free buffer-512, 0000000080da38b5/0x00000000fff8b600 (bad dma)
The difference between the two bootsequences is that with the bugged kernel, xhci times out at some point, dies, and then proceed to unload every usb devices. In my case, usb 1-5 (the second to last line in the log) is my internal trackpad / keyboard, and therefore I get stuck on the decryption prompt. The last line (bad dma) also only appears on bugged kernel.
The last kernel update also didn't fix the issue btw, if anyone is wondering.