Here is the seat bag we havehttps://ortliebusa.com/product/saddlebag-micro/
It sits nicely, slides out of its mount if needed but otherwise you just access it through the drybag-style roll closure. Waterproof. We got them for the same reason you are spotting: having a mounting strap around the seatpost may be an issue with dropper posts.
It is small but able to hold what you have listed. I have a 29er tube, 2 thick Pedros tire levers plus a third regular Zefal lever, very small multi tool, https://www.topeak.com/global/en/products/Mini-Too...
, chain tool, patches, tire boot, spare link and spare derailleur hanger in mine. That fills it pretty full. I could squeeze another tiny thing or two in there (or maybe a bigger tool), or maybe give the roll closure one less flip and still close it.
It doesn't bounce or wiggle. I don't notice it when I move the seat up and down but I am able to drop my seat low enough that I can rub the pack with the tire for a second if the suspension comes way up due to landing a large drop. This is because of my frame size versus my leg length; I barely have any of the stationary part of the seat post extending out of the frame, so most of my seat post that you see is droppable (I'm making up words. My vocabulary is a bit janky). I hear it buzz for a second if the tire ever rubs the seat pack but it still hasn't caused any wear to the pack, so it is a non-issue.
It does not have a clip for a rear light. You could probably attach one to the bungee loops on the side that you use to close it. Maybe not ideal for direct visibility for traffic etc but enough to satisfy race requirements.
I used the seat pack for WT and had a dog leash tow system attached under my seat as well. If you or anyone is curious, I'll have to remember how I positioned them both or look for some photos. It worked well and wasn't tricky.
FYI, I raced that bike at WT with a 1x drivetrain AND had reduced the gearing on the front ring, yet was still able to hold the pace on the pavement. Must have been the advantage of the soft, wide tires ;) (that IS what the Schwalbe research claims.... lower pressure and wider tires roll faster... except for on pavement). Truth be told, I was almost at my hamster cadence limit but still managed.
Quite happy with the seat pack. They make other sizes if you wanted something bigger for AR or joining Revy for a ride to Argentina.