Blender add-on for loading, saving and resetting shape key values.

LoadShapeKeyValues.py

bl_info = {
        "name": "Load Shape Key Values",
        "blender": (3, 0, 0),
        "category": "Object",
        "location": "Object Data Properties > Shape Key Specials",
        "description": "Load, save and reset shape key values",
        }

import bpy, json
from bpy_extras.io_utils import ImportHelper, ExportHelper
from bpy.types import Operator
from bpy.props import StringProperty

def get_object():
    assert bpy.context.mode == 'OBJECT', "Must be in object mode!"

    ob = bpy.context.object
    assert ob.type == 'MESH', "A mesh must be selected!"
    assert ob.data.shape_keys, "Object must have shape keys!"

    return ob

class LoadShapeKeyValues(Operator, ImportHelper):
    """Load shape key values from a JSON file"""
    bl_idname = "object.load_shape_keys"
    bl_label = "Load Shape Key Values"
    bl_options = {'REGISTER', 'UNDO'}

    filter_glob: StringProperty(
            default='*.json',
            options={'HIDDEN'}
            )

    def execute(self, context):
        ob = get_object()

        data = {}
        with open(self.filepath, 'r') as f:
            data = json.load(f)

        kbs = ob.data.shape_keys.key_blocks
        changed = 0
        skipped = 0
        for k in data:
            if k in kbs:
                if  kbs[k].value != data[k]:
                    kbs[k].value = data[k]
                    changed += 1
            else:
                skipped += 1

        self.report({'INFO'}, f"Changed {changed} shape keys, {len(data)} in file, {skipped} missing on object.")

        return {'FINISHED'}

class SaveShapeKeyValues(Operator, ExportHelper):
    """Save shape key values to a JSON file"""
    bl_idname = "object.save_shape_keys"
    bl_label = "Save Shape Key Values"

    filter_glob: StringProperty(
            default='*.json',
            options={'HIDDEN'}
            )
    filename_ext = '.json'

    def execute(self, context):
        ob = get_object()

        data = {}

        for kb in ob.data.shape_keys.key_blocks:
            if kb == kb.relative_key: continue
            data[kb.name] = kb.value


        with open(self.filepath, 'w') as f:
            json.dump(data, f, indent=4)

        self.report({'INFO'}, f"Saved {len(data)} shape keys to '{self.filepath}'.")
        return {'FINISHED'}


class ZeroShapeKeyValues(Operator):
    """Set all shape key values to zero"""
    bl_idname = "object.zero_shape_keys"
    bl_label = "Reset Shape Key Values"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context):
        ob = get_object()

        for kb in ob.data.shape_keys.key_blocks:
            if kb == kb.relative_key: continue
            kb.value = 0

        self.report({'INFO'}, 'All shape keys were reset.')
        return {'FINISHED'}

def draw(self, context):
    self.layout.separator()
    self.layout.operator(LoadShapeKeyValues.bl_idname)
    self.layout.operator(SaveShapeKeyValues.bl_idname)
    self.layout.operator(ZeroShapeKeyValues.bl_idname)

def register():
    bpy.utils.register_class(LoadShapeKeyValues)
    bpy.utils.register_class(SaveShapeKeyValues)
    bpy.utils.register_class(ZeroShapeKeyValues)
    bpy.types.MESH_MT_shape_key_context_menu.append(draw)

def unregister():
    bpy.utils.unregister_class(LoadShapeKeyValues)
    bpy.utils.unregister_class(SaveShapeKeyValues)
    bpy.utils.unregister_class(ZeroShapeKeyValues)
    bpy.types.MESH_MT_shape_key_context_menu.remove(draw)

if __name__ == "__main__":
    register()

To install, save this file somewehere, then in blender: Preferences -> Add-Ons -> Install... and select the file.