Our old house was freezing so little one used to be in fleece sleep suits, 2.5 toG sleeping bags and cellular blankets. However, if you go on the gro website there are chares which tell you advised clothes for baby at that room temp.
The general advise is its better not to have extra blankets if using sleeping bags (as kind of defeats the purpose if you do as they can then cover babies face)
So for example at 18c it might be long sleeved vest plus vest with 2.5 tog bag. Then is might be 19c short sleeve vest only with 2.5 tog bag etc....https://gro.co.uk/pages/what-to-wear
Check what the minimum weight for any bag you have is tho. My DD was born 8 lb 2 oz so she was fine going straight into the bags as the ones we had have minimum weight of 7 lb. The critical thing is actually that their head cannot fit inside the bag when done up. So if when done up, the bag fits up over their neck so in theory that baby cold wriggle around and end up with mouth inside the bag then they cannot use that bag yet. Some manufacturers have different size neck holes so something to think about when buying them.