I understand your plan but I'm not sure what design market you aim to satisfy. What type of design are you intending to do ie domestic, commercial, industrial, PLC, HMI, BMS, systems interfacing etc? How much of this design work have you been involved in for clients in the past and how big a market have you identified?
As for your existing business, if you can't integrate the design work and your existing customers or run both enterprises in parallel then I'd aim to sell your existing business if you're happy you've done sufficient due diligence into your new design business and it will be profitable.
You'll need to set your old business up with a new structure that will run without your involvement for it to have any value. You'll have to employ an electrician on a proper employment contract and over a few months familiarize him with the customers and the equipment/installations to the point where you can send him whenever they call and they're happy with that. At that point your existing business could be a salable entity that has some value because the employee would go with the business when it's sold. TBH the employee could be the best candidate as a buyer so maybe look for someone with that in mind as an end game.
Goodwill alone rarely translates into a salable item in my view because with most small businesses the customers aren't employing 'the business' they're phoning 'you personally' because they know you, like you, trust you, have a longstanding relationship with you and I as a 'buyer of the goodwill' can't provide them with this because I'm not you. All the restraint of trade agreements in the world and all the introductions and schmoosing still won't be able to persuade the customers that I'm you so invariably 90% of them will make other arrangements to source another service provider. From experience as well I've had several business owners wanting to 'sell the goodwill' of their business in the form of a customer list and a good word from them and I've never been tempted. Almost guaranteed I can harvest the best of their customers myself over the following months via the normal marketing channels and without parting with the ridiculous price they've valued their goodwill at.