Hi Jan, 
I- what you want is: "When a quotation is created, the related opportunity must have the stage X."
the algorithm is pretty simple: 
1- when quotation's create funtion is triggered.
2- if field opportunity_id is not empty, then trigger lead stage change.
3- if not, standard quotation creation.

the code should be something like this: 

@api.model
def create(self, values):
    rec = super(....).create(values)
    if rec.opportunity_id:
        rec.opportunity_id.stage_id = Ref to stage X
    return rec

II- If the process should be triggered while calling an automated action, then it's almost the same logic:
1- If automated action is called
2- Basically it will create quotations i assume ? 
3- you get all quotations created and say :

quotation_ids.mapped('opportunity_id').write('stage_id': Ref to stage X )

Hope this helps.
Upvote if it does.