MediaWiki 1.32: Error creating thumbnail: Unable to save thumbnail to destination

Question

I've been running MediaWiki 1.19 on Fedora 24 for ~3 yrs with zero problems. Unfortunately my Linux box died, so was forced to move to MacOS + MediaWiki upgrade. It hasn't been fun.

I've finally got everything working, except for this one problem. I can upload an image, but then thumbnail creation fails - a sine qua non.

MacOS 10.13.4
MediaWiki 1.32.2
Apache/2.4.29 (Unix), Jan 17 2018 18:20:31
PHP: 7.1.14 (cli) (built: Feb 7 2018 18:33:30) ( NTS )
MySQL: Ver 15.1 Distrib 10.4.6-MariaDB, for osx10.13 (x86_64)
ImageMagick: 7.0.8-50 Q16 x86_64 2019-06-25

LocalSettings.php:

$wgScriptPath = "/mediawiki-1.32.2";
$wgResourceBasePath = $wgScriptPath;
$wgEnableUploads = true;
$wgUseImageMagick = true;
$wgSVGConverter = 'ImageMagick';
$wgImageMagickConvertCommand = "/usr/local/bin/convert";
$wgUploadDirectory="$wgResourceBasePath/images";
$wgTmpDirectory="$wgResourceBasePath/images/temp";
...
$wgShowExceptionDetails = true;
$wgDebugLogFile = "/tmp/mediawiki-errors.log";

ImageMagick is definitely present and correct, tested thusly:

convert test.png -quality 80 -background white -define jpeg:size=112x120  -thumbnail '112x120!' -depth 8 -sharpen '0x0.4'  -rotate -0 test.jpg

$PATH shows the location of convert correctly. Changing convert (now deprecated) in favour of magick in LocalSettings.php makes no difference whatsoever.

In desperation, I've chmod -R 777 mediawiki-1.32.2. (I'm aware this isn't okay. It is purely a debugging exercise.) On uploading a file, a subdir is successfully created, eg:

-rw-r--r--  1 _www  staff  128069 26 Jun 19:28 testing-stuff.png

But no sub-sub-dirs are created for the generated thumbs. This leads me to believe the problem lies with ImageMagick, ie convert. The log file doesn't mention convert at all, just moans cyclically:

File::transform: Doing stat for mwstore://local-backend/local-thumb/b/b9/testing-stuff.png/500px-testing-stuff.png
[FileOperation] FileBackendStore::getFileStat: File mwstore://local-backend/local-thumb/b/b9/testing-stuff.png/500px-testing-stuff.png does not exist

I really hope someone can help. I'm at my wits end.