As it's starting to get cooler, my 7month baby is waking regularly. Currently room is 18 degrees and she wears 3.5 ergopouch sleeping bag and bonds wondersuit. I will add more layers as the weather continues to get cooler however I'm just wondering about the addition of a merino wool mattress underlay to help keep her warmer? Is this safe sleep recommended? The one I'm considering is from baby bunting.

Red Nose recommends that babies sleep on a firm mattress.
Any mattress protector or underlay that takes away from the firmness of the mattress, is not recommended.
https://rednose.org.au/article/what-is-a-safe-mattress
Babies do wake when they are not warm enough but increased frequency in waking has many causes.
Have you had a feel of her back/tummy when she wakes .
“A good way to check baby’s temperature is to feel baby’s back or tummy (don’t worry if baby’s hands and feet feel cool - this is normal).
Consider how many layers that you as the baby’s carer are wearing comfortably.”
https://rednose.org.au/article/how-to-dress-baby-for-sleep