Only adding to this because it's likely a hardware failure, and I had a challenging time debugging a similar issue in an aftermarket engine ECU years ago. Thought it might make a fun anecdote.

The car would run fine once started, but the car just wouldn't start sometimes (quite modified so I knew the systems well). The started would turn as that was a simple relay, but all ECU controlled devices wouldn't trigger. Plugging into the ECU, no error codes and all looked normal.

Eventually we tracked the issue to some corruption in the ROM that was only getting read in certain circumstances, since the ECU stores maps for engine parameters based on things like pressure and temperature you might only hit the corrupted bits of a table in very specific circumstances.

Reflashed the ROM and all was good afterwards. The suspected cause of corruption was intermittent power supply that had been fixed a while earlier.