You can draw essentially on any canvas and croppy will auto format it for you into the webtoon format- especially if you use the webtoon croppy extension.
Like I know people that work on canvas that is 1000 x 10000. or 2000 x 5000 or whatever they feel like. And you can use different file types including png, tiff, psd and croppy converts all of those. I think the only file it doesn't work on is the medibang files and the Clip studio file.
I personally draw all my panels sep and copy paste them onto a 800x20000, save as a png and then I let croppy do the rest- it will also convert the same file to fit Tapas too. so you don't need to make separate files.