We rent out a seaside beauty and get 45 weeks a year. We use "Out of Eden" who supply the hotel industry and their towels last 2/3 years. We use blue, not white.
But, it HAS to be spotless. No grime anywhere. Cooker clean and grease free. No dust. Bathrooms spotless. Beds made. No cracks, broken things, light bulbs blown. Everything perfect. It doesn't have to be the highest quality. We use a mix of IKEA, John Lewis and odd bits and pieces but it has to be clean. People arrive and go, "Aah, lovely" and then give you no trouble. If they arrive and find a pube in the shower, they will then work to uncover any other flaws.
Market it using rustic, rural, charming - not luxury, perfect...
Allow dogs (2 times the number of bookings) have a travel cot and high chair. Have a wood burner or open fire. All these make a difference. Listen to feedback, act on it. Immediately offer refunds / apologies etc as your reputation is everything.
Look for family bookings. Post photographs of every room.
However, people don't want a holiday cottage anything less than their own home: they don't want 8 people queuing for one loo. They want everyone to have a plate, knife, fork, seat in front of the telly, dining chair etc.
My advice would be to not go down the sleeps 4/6 with a sofa bed either. Make it a comfortable 4 rather than a squeeze for 6. People forget they have saved money when they get there and just notice the lack of space.
Pm me if you want more help. We have been in the game a long time. We also have a city centre place but have to have much more "boutique hotel" style furniture and fixtures and fittings - it is what is expected in the city.
Oh, and buy a really, really good Hoover.