Defence 4

I have a Carinthia Defence 4 which is fantastic but quite a ways out of your budget. But the Army issue medium weight bag is a direct copy of it just without using the G loft down. Comfort down to -10c and you'll be able to get one within your budget.

Thermal undies, preferably merino wool. Carinthia sleeping set up - Defence 4 sleeping bag with Truepin sleeping bag inside. Cover these with a good quality bivvy bag. In such cold you will definitely need a **4+, R-rated** sleeping mat. Maybe invest in some hand-warmers and a hat.

If your intended use case is sleeping outside in winter, you will die in one of those. Buy proper sleeping bag, like https://varusteleka.com/products/carinthia-sa-m05-makuupussi-oliivinvihrea-fi.

If you want simple, low-ish price range and reliable/robust, I can recommend Carinthia Defence 4. It fits your requirements almost - it is not small though. Personally I think it's comfortable although it probably lacks some comfort features.

Get something that is used by military. I can highly recommend Carinthia (I own MIG 4 jacket and it’s a great piece of kit) This bag has synthetic filling and it’s water repellent with comfort temperature -9 degrees https://military.eu/en/p/carinthia/carinthia-defence-4-large-sleeping-bag-olive-2212626?pref_q=carinthia+defence+4+large+sleeping+bag+-+olive

I prefer something that is robust and reliable. It’s not like you’re going to carry it for weeks

Carinthia Def 4 and for summer the Carinthia Tropical (both can be combined for more warmth). The Def is usually available at army surplus while the tropical is more rare. In the US the US extreme cold weather sleep system might be easier obtainable.

Carinthia Defense line. Goes to -30 degrees celcius (or more...) and won't break the budget.