I've been looking myself (Florida) and I'm starting to think silk filled is the way to go. I've also read good things about wool filled, but I'm leaning towards silk as they're cheaper. Wool just screams winter to me. That said, in the meantime, I've been using a thick cotton blanket or my duvet insert that's not filled and honestly they've been very comfy. More on reddit.com

This is the only insert I can stand in the hot summer months, and even then I usually just use it like a quilt without the cover because I don't need the added layers of warmth. More on reddit.com

No, you're correct that there is nothing that is fluffy/puffy like down that is cool in summer. Such a thing doesnt' really exist and the only thing that can come close to it in terms of 'fluffiness' is silk. Silk all the way if you want something that isn't too hot during summer. Down is just not practical for summer unless you're cranking the AC on at 70F and below temps (even with ultralightweight down as u/MaximsDecimsMeridius says). I would rec a summer weight silk insert, it will be thin but it'll keep you cool like a champ. Good brands are: St Geneve (expensive), THXSilk, Mommesilk, ElleSilk and DaniaDown (if you want larger sizes like Super King). Wool in summer weight will also work pretty well, though I feel that silk is the better option in terms of breathability for summer but wool tends to be even thinner than silk can get in summer weight. More on reddit.com

2 twin comforters is a pretty common solution to different sleep preferences on larger beds. sleep is important and there's no rule stating a couple has to sleep under the same single blanket or that it means you dont love each other. you can also get one large lighter comforter and you can each layer on quilts or blankets on top as needed. in terms of a less insulating one, try looking for "summer weight" or all season. summer weight being cooler obviously. for reference, i sleep in the 68F to 71F range and i hover around summer weight (if its insulating down) to all season range (silk). if the insert shifts, its the wrong size for cover and its too small. typically, inserts should be a couple inches smaller larger than the cover so they dont shift around inside the cover and if not, corner ties might help. fwiw, all of my king size inserts dont have issues shifting around inside my king size covers because of this, sometimes i have to order cal king instead of regular king depending on cover size and insert size. i had a cover that was a few inches larger than my insert and the insert had horrible issues shifting around. its why my wool insert doesn't get used, its too small and shifts around (100 x 90 in insert size, 104-108 x 92-94 covers) Edit: yea the insert should be bigger, not smaller. More on reddit.com