Looking for advice on how/if I should approach cross platform development.

I want to make a commercial application and have it be easily accessible to as many people who want it. I dont want to limit sales from accessibility. I'm familiar with WPF and C# mostly so I would prefer to stay in this area. I looked at Avalonia and my only concern is that it's very new.

Basically I'm looking for general advice on how people approach this. There doesnt seem to be much cross platform stuff for C#, is switching to Java a better option? Is it even going to limit sales that much by only developing on Windows? How do companies develop in this case? I tried doing my fair share of google research but I'm not finding good answers.

If you have any advice that someone looking to develop commerical software should be aware of feel free to post it. Thanks.