This has been my life for getting onto a couple of years now. Subsequent planning for the conversion works post the establishment of C3 via Class Q has been a long process due to a lot of local opposition.
With that in mind, the way that ours got over the line was via the class Q change of use (In our case from B8 Agricultural storage). Based on this experience i would absolutely certainly establish the use class using class Q if you can meet the tests to satisfy this. As you surely know, this is an extension of permitted development, and if you do meet the tests then you will receive acceptance via prior approval which will set the principle of residential dwellinghouse use (importantly including the suitability of the building to be converted, case law in place which tested this, an open metal barn was the case and was rejected as not being suitable for conversion without major works requiring planning permission).
The entire application for ours was underpinned by the changed use class being established, and thus it is inevitable that the building will be being converted to a dwellinghouse. At this point the planning process for the conversion works themselves centred over mitigation to the local environment, rather than the principle as to whether a dwellinghouse may be established in the location.
Straight planning for a change of use and development will surely be flat out refused? I don't know about the location in question here, but if the use class has never been C3, and this building is in open countryside, then there seems to me to be very little chance that you would ever change the use via planning permission....
In our case, a hamlet, a case of simply not wanting additional development to an otherwise very quiet place.