Hello,
It is possible to make few fields read only based on any indicator. I believe you already have an indicator value set to true based on the desired condition when you want the standard fields as read only. You can go to extensibility explorer and select the corresponding section where the fields are added and if you have the option "Adjust Properties" for that section , you can bind the read only properties of desired fields to the indicator.
Note: Sometimes few fields may not have the option to change the read only property.
Hope it helps.
Thanks & Regards,
Meghna