Try to limit previews size in config.php to e.g. 1024 (basically if you need bigger, you can download original image already). For me it works fine, it reduced previews folder from 20+ Gb to 6Gb.
'preview_max_x' => 1024,
'preview_max_y' => 1024,
More about this config under https://docs.nextcloud.com/server/14/admin_manual/configuration_server/config_sample_php_parameters.html#previews
You can also added occ preview:pre-generate to cron to create new previews automatically.