I’d use a professional mechanic if a) it didn’t cost a fortune, b) I could get things done as and when I need them doing, and c) I could trust that it isn’t going to be bodged.
As with any job you are able to do yourself, it seems to expensive to pay for someone good to do and at the price point you are willing to pay, it will almost certainly be done badly.
Doing internally routed cables is just a fun Sunday afternoon puzzle, it’s all about mindset! The cable routing on a giant Defy is almost comical in the sharp angle for the front mech and ridiculously difficult portion through the chainstay for the rear mech.