$GLOBALS['TCA']['sys_category']['columns']['description']['config']['richtextConfiguration'] = 'mini';

This does not work when PageTSconfig RTE.default.preset is configured. You'd have to set the following Page TSconfig in this case:

RTE.config.sys_category.description.preset = mini

Alternatively, do not use RTE.default.preset but override the default preset in ext_localconf.php:

$GLOBALS['TYPO3_CONF_VARS']['RTE']['Presets']['default'] = 'EXT:extname/Configuration/RTE/Mini.yaml';