I think I broke my upgrade

I was performing the upgrade to 1.6.3 from 1.5.10 (I think) on my Dreamhost space. In order to do so I also needed to set it to use PHP 7.2, but in the process I think I managed to break the upgrade. Now the site is down, and I can't even upgrade from the shell as it throws errors with the AbstractLogger?

[myserver]$ bin/gpm self-upgrade -f
PHP Warning:  include(/home/foo/bar.org/vendor/composer/../psr/log/Psr/Log/AbstractLogger.php): failed to open stream: No such file or directory in /home/foo/bar.org/vendor/composer/ClassLoader.php on line 444
PHP Warning:  include(): Failed opening '/home/foo/bar.org/vendor/composer/../psr/log/Psr/Log/AbstractLogger.php' for inclusion (include_path='.:') in /home/foo/bar.org/vendor/composer/ClassLoader.php on line 444
PHP Fatal error:  Class 'Psr\Log\AbstractLogger' not found in /home/foo/bar.org/vendor/maximebf/debugbar/src/DebugBar/DataCollector/MessagesCollector.php on line 20

Similar errors show in my server logs for accessing the main page.

Any help gratefully received :'(

In the end I gave up and overwrote everything with a fresh download. Once I'd grabbed a couple of the relevant config yaml files, everything seems to be back up and running.

Although..... getting the 500 error red box on the dashboard and upon saving pages (and possibly elsewhere) which shows in the logs as:

Premature end of script headers: php72.cgi

Seems to be a relatively generic error, but without a generic fix. Anyone else seeing this?