Firmware misbehaving after PIO IDE update

Greetings all, first off very grateful for this wonderful tool and have been enjoying using it to sporadically battle with my old 3d printers

I’ve been building and uploading Marlin 2.0.5.3 to my bank of CTC Bizer (Mightyboard REV H, Mega 1280) printers for a while now with great success, but I mistakenly accepted an update to PIO from within VS Code today, and after a bit of fiddling, I can build and upload again, however there is an issue.

Upon rebooting after upload, the printers all do the same, the Marlin splash is displayed, then they the screen stays illuminated but goes blank.

The printer does not respond to commands over USB so it seems there is some kind of crash when booting the firmware.

TBH, not sure what version I had before but would live to downgrade and try to get back my previous setup, but having trouble. I saw it mentioned elsewhere I could downgrade to v5.x by doing the following in the PIO CLI shell:

pip install “platformio<6”

This seems to work out, but then PIO home still shows Core 6.15, and the firmware still fails to work when uploaded. I started by trying to install earlier versions of the VS Code plugin, but that didn’t change the Core version - I’m stabbing in the dark here I’m afraid. I’m using Autobuild Marlin FWIW, but it seems that is there mostly to stage the platformio commands.

If there’s a better way than downgrading, I’m all ears and willing to try to diagnose the issue, but feeling a bit lost and would be very grateful for any kind suggestions

Cheers