imho you bought the wrong shower. I would have fitted at home one which is accessible for servicing like a surface mounted round style one. Point elbows back at wall, first fix leave 2 15mm stubs sticking straight out of wall. Then entire valve with filters and all can be removed for changing cleaning new insert anything you like.
If custards are stupid enough to want something which can't be serviced I simply point out their stupidity. If after I have called them stupid they still want me to fit it then I go ahead. Usually a few weeks later something breaks because most thoings are so badly engineered today.
But customers are sweet, because they are eternal optimists. I like that.
But I don't like valves that stick out of the wall, they look untidy (in my opinion), lazy and tacky - inbuilt showers may be a hassle if they go wrong, but worst case scenario I will only have to remove the front siliconed plate and one tile, not the end of the world.