Data repair and upgrade after update
> MediaWiki requires the PSR-3 logging library to be present. This library is not embedded directly in MediaWiki's git repository and must be installed separately by the end user. Please see mediawiki.org for help on installing the required components.
This message is generated by MW itself and has not really something to do with SMW (whether you would have installed it via Composer or the Tarball).
In regards to the message, if you run
composer update from the MW base directory then it should install the libraries as requested by MW.
> Having to go AFK now, bit surprised how difficult this process has been, I had it up and running quite well before.
Looking at [0, 1], I personally find the process fairly simple but it varies with experience. Now (as of 1.25), that MW uses Composer itself to manage external libraries, SMW is just one component to leverage the infrastructure necessary in handling external dependencies.