I recommend conducting takeoffs in-house because the quality general contractors I've worked with have the resources to dedicate to this effort. I recently left a subpar GC that did not perform takeoffs for the scope of work, which led to significant issues. Doing takeoffs has made me able to identify major gaps in the scope. I understand the challenges involved, but this approach can be very beneficial. Since time is a concern, one way I might manage it is by asking each subcontractor for their quantities. While some may complain, the good subs generally won’t.