Hello, my baby currently sleeps in a love to dream grow bag with a 3.5 TOG as her room goes below 16 degrees at night. Just wondering what is the most appropriate material onesie to put her in for underneath? She is 6 weeks old.

There is no specific amount of clothing/bedding for baby to wear. all babies are different.
Red Nose recommends the following:
Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.
Dress baby for sleep and add/remove lightweight blankets to ensure baby’s back or tummy feels comfortably warm to the touch.
Remove hats, bonnets, beanies and hooded clothing from baby’s head as soon as baby is indoors.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep