compress, convert, update JSON. quick.
ZapArtist is an application used for automating the workflow for creating a WhatsApp sticker. With a 512x512 PNG, you'll be able to obtain a compressed and ready for WhatsApp WebP. It'll also attempt to insert the new WebP in the configuration JSON, relieving you from the pain of editing the file manually.
...\ZapArtist\bin\Release> .\ZapArtist.exe <path to PNG image> <path to directory containing the configuration JSON> <sticker pack name>
Disclaimer: There is no functionality for creating a new sticker pack in the JSON file. You should probably use an existing one, and if needed, create one manually. :(