I have been a long time Windows user since the first version came out in the late 80's. I made the switch to Linux back in 2014 for a number of reasons. There are many ex-Windows Users that are looking for a good Photoshop and Dreamweaver replacement for Linux.
Affinity Photo looks like it can fill the Photoshop void in Linux.
Linux Users are not adverse to paying a reasonable amount for quality software that works. Your current pricing for the other platforms is reasonable and a good price point.
I also recommend that you launch a Kickstarter campaign to test the Linux market. I believe you will be surprised to see actually how many Linux Users that are hungry for your photo editing product. If you do start a Kickstarter campaign to create a Linux version be sure to let everyone on this forum thread know for we will help spread the word.
Someone mentioned Foundry products in another post. These are high-end applications for a niche market. Photo editing has mass appeal.
You may be interested in developing your apps with QT so that they run across all platforms. Here is their link.
https://www.qt.io/application-development/