Well, if you have been to the Lake District before and don't particularly want to spend much time there, you could maybe head straight for North Wales and spend the extra time there - maybe spend 2 nights there so you get a full day in the area?
The drive from North Wales to South Wales (if you take the A470 through mid Wales) is highly scenic, so that is worth keeping in your itinerary. The alternative would be take the English borders route and to drive down through Shropshire / Herefordshire which is also rural and pretty, but less dramatic. We've had a few holidays / weekends away in that area and like it, but it is fairly low key.
It might also be worth taking into account which days you will be doing the motorway drive. Fridays (afternoon /evening in particular) and Sundays (late pm / evening) can be particularly bad on the motorways. Also Saturdays during August would tend to be busy for that part of the M6 motorway.
With friends I once did the journey from Glen Coe in Scotland to Cardiff over two days (Sat - Sun with an overnight stay with friends in Yorkshire, so not the most direct route I suppose). We did do a few quick stops on route, but I found it quite gruelling. But then I am not a fan of long drives!
You may be absolutely fine doing the trip in 5 days if you are used to lots of driving.