Ionic applications usually target mobile so testing your app via VoiceOver and TalkBack will cover most scenarios. However, you can target the web in general as well and this means that your users may be on Windows, Mac or Linux desktops and laptops.
Use analytics from your app to see what devices your users are using and test appropriately. You can also take a look a the WebAIM Screen Reader Survey to get an idea of readers used.
Here are some popular choices: