I’ve had my bassinet mattress custom made and it’s about 10cm thick, I read that you recommend 7.5cm. Why is that? And would I endanger my newborn by having him sleep on the one I’ve got?

Red Nose recommends that all mattresses for babies should be firm, clean, well-fitting & flat (not elevated)
Is your mattress firm & does it fit your bassinet snuggly. (ie no gaps or “curling” at the edges.)

Product Safety Australia develops safety guidelines & standards.
This is where the 7.5cm thickness for a bassinet mattress comes from.
While there has not been Mandatory standards for bassinets up till now, Product Safety Australia (ACCC) are in the process of formalising this & their Safety Standards will be Mandatory from Jan 2026.
In the meantime the following is their recommendations (buying tips)
https://www.productsafety.gov.au/consumers/keep-baby-safe/settle-baby-to-sleep-safely/bassinets-and-cradles-guide

The thickness of the mattress needs to tie in with the height of the sides (should be at least 25 -30cm from the top of mattress) to reduce the risk of a baby falling from bassinet.
What is the height of the sides of your bassinet?
https://rednose.org.au/article/bassinets