Hi AUZEMERY Vincent,
We would like to inform you that to create rounded corners on the header with an icon inside in SfAccordion, you need to disable the default header icon by setting the "HeaderIconPosition" property to "None." Then, add a custom image icon using a converter based on the "IsExpanded" property. For your reference, we have attached our sample.
UG link : https://help.syncfusion.com/maui/accordion/appearance
Regards,
KamalaKannan S.
Attachment:
SampleAccrodion_79d52744.zip