You want the field to be Readonly, but trying to add value through Onchange or so, then you need to under-go hassle of updating for the same again in the Create/Write ORM methods.

Instead the feasible alternate solution would be you to use Compute/functional fields.

Api decorator lets you add depends(dependency) field, which acts as both onchange and computational method, that way you don't have to worry about manually re-updating the same in ORM methods.

Note:

If a field is marked as Readonly, odoo will explicitly omit retrieving data for the same from UI, hence in such case its best to use computational/related fields.

dear Karan Gupta

you need to override create and write method and assign the value again by calling your onchange function and then update the vals.