I don't know about EU availability, but I've slept out in those temps in a OneTigris "4 season" underquilt https://www.amazon.com/gp/product/B08938G2DS/ which is ~40€. I put 4 season in quotes because it's totally a 3 season UQ.

I went out with just the UQ in freezing temps and that was not nearly enough, but combined with the cheapest of the cheap closed cell foam sleeping pad, a 3-season sleeping bag and moderately warm clothing (basically what you'd wear in freezing temps to go grocery shopping rather than what you'd wear to go hiking and sleeping) and I slept pretty warmly.

The pad alone wasn't nearly enough and the UQ alone wasn't nearly enough, but combined they were pretty good.

One big factor that doesn't seem like it would be much but makes a huge difference is how your pitch your tarp. Mostly closing off the ends and pitching low and tight to the hammock makes a pretty big difference.

But all of it together was heavy and bulky. If you're going to be hiking for the whole week-long trip, you're probably better off buying something more expensive than can function without layering, packs smaller, and is lighter.

Thus the eternal compromise -- cheap and heavy or expensive and light.

I've used a Onetigris underquilt for years, and have been very pleased with it. I see they have a couple different sizes now, cheapest one is $40