https://perfectlysnug.com/products/the-smart-topper

Our son swears by this system.

100% linen - I have temperature control fluctuations. I have many 100% linen duvet covers - sometimes I sleep without anything inside the duvet cover (during summer). I have a nice silk duvet insert for when it's cooler + a down insert for winter. However, right now I've just been still using my silk duvet insert.

While the down insert provides that fluffy/cozy feeling, while the silk insert doesn't provide as much, the silk seems to work in quite a range of temps. It has to be a good silk insert + has a 100% cotton cover over the silk.

FYI - I use 100% cotton percale fitted sheets - no top sheet needed with a duvet cover because it's changed just like sheets.

I have a few linen and cotton coverlets - but I tend to just fold them down to bottom of bed and don't use them to cover myself.