First, 300 dpi has been an industry standard forever. I don't understand the inflation of DPI to resolutions such as 600 dpi. Most of the details at that resolution will probably not be noticeable when used for print.
Even for line work, a crazy resolution such as 600dpi and above is overkill. While it could be used for a working file, to afford as much wiggle room, beyond 450dpi, you will see very little difference in print and it will inconvenience the production. Working with larger files is slower and requires more computing resources.
What has not been mentioned above is that DPI matters mostly if you work with raster (bitmap) files. If you work in a vector format, you will need to export to a suitable format and DPI. Vector illustration gives people more flexibility for print.
The DPI needed for computer screens, television, and film is not 75. It is 72 dpi. If publishing a web comic, always export your image to 72 dpi, regardless of the actual size and format of the comic. Anything above that is overkill and will slow downloads for users.
I'm not sure how you arrived at 2778x4167. I would stick with existing comic formats based on the genre and market you are targeting. For example, if publishing a standard North American comic, 12x18 (actually 11x17 but it gives you a bit more wiggle room than standard tabloid size) inches is fine. Manga and European comics have different formats.
Based on the format, calculate the translation of 12x18' into pixels at your working resolution.
In Photoshop, create a new file.
Select the US format (for tabloid).
Select tabloid (11x17).
Adjust to 12x18 if you want. The resolution will be 300 dpi by default.
Adjust it to 450. If you do you will notice that your file size estimate will go from about 48mb to 108mb. 108mb is not usually a comfortable working size. It will be slow. That's why 600 dpi and above is excessive.
You will notice that the pixel ratio is now 4950 x7950 at 450 dpi. At 300, it would be 3300x5100 for an American comic.