In terms of sleeping cooler while still maintaining fluffiness, it's a tough one. Down alternative (synthetic) is just not as breathable as natural fills. Opting for something with a cotton shell will help marginally, as cotton is more breathable than polyester. At some point, you may need to choose between fluffiness / weight and warmth and see which one you're willing to compromise on more. When shopping across brands, I would compare specs such as shell type, shell thread count (higher TC = softer), fill type (microfiber, polyester batting, or other?), fill weight, etc. You should be able to get something good quality within your budget, it's just about finding the specs that work best for your needs. Also worth checking if the brand offers a good return policy and/or warranty so you don't have to stress too much pre-purchase. In terms of natural down, it's a tough call. Down is a by-product of the meat industry, and you can find many companies that source their down ethically and post-mortem, with certifications against things like force-feeding, overcrowding, lack of vet care and other inhumane practices. But if you are opposed to eating meat and the meat industry period, these assurances may not go far enough for you, and that's ok. You're right to consider sustainability - not only is down a natural product, it should last you a lifetime (or longer) and can help reduce home heating costs in a cool climate. In terms of your concern about leakage, make sure that the company is using down-proof cotton and that they offer a warranty in case of any defects. There shouldn't be a lot of dust coming out, ever (given proper care instructions have been followed). Commercial down is only allowed to have a minimal amount of dust in it. It's impossible to remove every bit, but the threshold is quite low. Make sure to purchase from a reputable company that has processes in place to test their down regularly for cleanliness. More on reddit.com

I have this one from ll bean and I love it! It stays fluffy More on reddit.com

I do not have personal experience with saatva, but this sub is pretty critical of the internet/instagram bedding vendors. The products aren’t awful, but they are generally not very good value for what they charge. At a quick glance it looks like their sateen sheet set is around $300- for that amount you can get a much, much nicer sheet set from one of the heritage linen companies like sferra or frette if you are willing to trawl ebay. The downside is the overstock vendors that sell those sets generally (though there are exceptions) do not allow returns for anything other then defective merchandise, so you need to know what you want in advance. If you decide to go this route i strongly recommend getting a bunch of fabric swatches first, so you know exactly which product lines you like the feel of. Most high-end companies will send them out for free More on reddit.com

What's your budget? You can get a decent-enough set for under $500 that will be serviceable but if you want really nice bedding it'll definitely be over $500, especially for the sheets. Let us know and we'll give you some recs that will do well for your circumstances, though I would definitely recommend you feeling (at the very least) the sheets in-store if you can rather than go off internet-only recommendations/brands like Saatva, Cuddledown, Brooklinen, etc. --- most of the brands you listed are mid-tier and won't really wow you, imo. More on reddit.com