Since barely no one actually replied to the requirements that you asked for I will do my best to answer them:
1 - The obvious distro that you would have to target/develop for is Ubuntu. Ubuntu is Debian-based but there's plenty of other distros based on it. The most popular distros besides Ubuntu are usually based on Ubuntu (ex. Mint). So if you develop for Ubuntu, your software would automatically be compatible with the vast majority of Linux distros.
This brings other advantages because Ubuntu offers something that no other Linux distro does: predictability. Every two years there's a Long-Term-Support release. This LTS release is then installed in servers, end-user PCs, etc. It is THE base for two years for the majority of Linux distros and it offers stability. Also, the second most popular Linux distro - Linux Mint - is based on Ubuntu LTS releases and they don't change their base for those two years. Effectively, Ubuntu LTS releases are the core for most Linux users.
2 - With desktop topology, I assume you are mentioning Desktop Environment? Well, if so you are in luck. Canonical has discarded their Unity desktop environment and is again using the main-line Gnome desktop environment. Therefore, you should support the Gnome Desktop Environment.
This is the obvious choice since it is the base desktop environment of Ubuntu 18.04, but its development is heavily funded by Red Hat (now belongs to IBM). The only other alternative would be Plasma (based on Qt) but they are not supported by the two biggest open-source companies in the world (Canonical and Red Hat).
3 - Deployment (paid) platform where we would recoup our development costs. Someone else already mentioned this but you should definitely use the Snapcraft platform. Snaps are a package format which is a bundle of your app and dependencies that works without modification across Linux. Snaps are discoverable and installable from the Snap Store, an app store with an audience of millions. You can create snaps from apps you’ve already built and zipped, or your preferred programming language or framework (this includes C/c++, Java, Electron, Go, Rust, etc...).
The Snap package format is accessible by default in all Ubuntu installations starting from the Ubuntu 16.04 LTS, and directly available from the Ubuntu software store. Also, Snaps can be easily accessed even by non-Ubuntu based distros and you don't have to do anything to make it compatible to other distros since the dependencies required are bundled with the app, so no conflicts arise.
Also Snaps auto-update and work like app formats for other platforms (ex. .apks for Android).
Here's an infographic from Snaps
Canonical's Snap Store has apps - this includes paid proprietary apps - from major publishers. Examples include Plex Media Server, Windows Powershell (!), Visual Studio Code and Android Studio, Only Office, Slack, Mailspring, Spotify.
Proprietary graphics software is also being released as Snaps. This includes Polarr. As you can see their developers followed a strategy similar to what I outlined for you.
It also includes Gravit Designer.
Ofcourse, there'll be challenges and I cannot guarantee you that if you invest 500.000$ into this that you'll be able to recoup your investment. I don't know your financial health, nor your budget or 3-year business plan.
There's always a degree of risk in every investment. However, be aware that plenty of companies that launched their proprietary software in Linux thought it was a worthy investment. Namely, Valve launched Steam and they keep investing and improving the Linux graphics stack including OpenGL support, Vulkan support and D3D to Vulkan support. Many gaming companies still release numerous ports for Linux (the Total War strategy games from Creative Assembly, the Tomb Raider series, etc).
I cannot run your company but if you were truly interested, a solid strategy plan would have to be outlined. However, I am certain that if you approached Canonical they would support you on this endeavour since they would also be interested in having professional proprietary software in their Snap store. They would probably collaborate in marketing your software and spread awareness about Affinity in social media and websites with enormous following like OMGUbuntu! would advertise Affinity software endlessly. If you consider this investment not only as a product launch but as integrating your marketing strategy then it would probably justify the investment.
Hopefully my reply is enough to convince that this might be a worthy endeavour.