You can try to:
- add the "moodle/course:manageactivities" capability to the teachers role and
- in the course, change the permissions for all the activities you don't want the teachers edit (one by one) to prohibit the teachers role the "moodle/course:manageactivities" capability:
On this way, you must be able to configure a role for creating activities but non editing the existing ones. Besides, you'll need to allow only the assign:addinstance capability if you want the non-editing teachers will be able to create only this kind of activities.
Hope this helps to you!