http://e26whn2524322mkxb3cbyk27ev2ihhq2biz35hty7gzgsyrwrygq27yd.onion
However, that custom approach has not actually been necessary for some time: in version 24, Nextcloud introduced support for using Imaginary for preview generation. Imaginary is a microservice which uses bimg to perform a range of image processing operations on request. I missed support being added to Nextcloud, but have now finally got around to setting it up.