Hi @carem ,
Do you want to keep the Toggle control as Edit mode within the Display form?
If you want to keep the Toggle control as Edit mode within the Display form, I afraid that there is no way to achieve your needs in PowerApps currently.
It is an known issue with the Toggle control within Display form -- The DisplayMode property of the controls within the Display form would be overrided by that from your Display form. Even though, you set the DisplayMode property of the Toggle control within the Display form to DisplayMode.Edit, it would still be displayed as Text.
As an alternative solution, you could consider set the DisplayMode property of the Toggle control in your Display form to following:
DisplayMode.Disabled
then the Toggle control would be displayed as a disable mode as below:

In addition, you could also consider use Radio control to display your "Yes/No" field value instead of the Toggle control. I have made a test on my side, please take a try with the following workaround:
1. Unlock the "Yes/No" field data card in your Display form.
2. Remove the original Toggle control, instead, add a Radio control within it.
3. Set the Items property of the Radio control to following:
["Yes", "No"]
Set the Default property of the Radio control to following:
If(
ThisItem.'Yes/No Field' = true, "Yes",
ThisItem.'Yes/No Field' = false, "No"
)

Best regards,