If you’re going to use the chain whip you just made, you’ll also need a way to get that cassette off. A lockring remover is a must-have; unless you use one of the shady removal methods I’ve seen on the internet—which I wouldn’t necessarily recommend unless you don’t care much about the condition of the wheel after you use them. I recommend Park Tool’s iterations here that cost about $6, but you’ll need to buy the specific one for your brand/type of cassette. (By the way, I’m assuming you already have an adjustable wrench laying around somewhere to use with this tool. If you don’t, well, you’ll need one of those too.)
As you mentioned, it is surprising how often you can get multiple flat tires on while riding, particularly on longer sprints. The tip about bringing a patch kit in your bike repair kit can be incredibly helpful if you’ve already had one flat tire. I agree that all you listed seems like an awful lot to carry, but it’s always better to be safe than sorry, isn’t it?
But before I get started, let me share with you a few tips. OK, one tip: Get a work stand. Because putting your bike upside down isn’t good for it—yes, I said it. When I first started fixing my own bikes, I couldn’t afford a fancy $300, folding, magnetic, fixes-the-bike-for-you stand. I tried piecing together a “DIY $30 Stand.” I followed all the directions that the Internet demanded, and once I’d bought all the clamps and pipes, this thing cost really more in the neighborhood of $60—but it only stayed upright with a rope tied to something sturdy in the garage. It was one word: junk. So I bit the veritable chainring and charged a decent stand on my credit card (I hate debt, folks).
It has some useful information for basic repairs... not so heavy on the maintenance side but there is some in there. The pictures and drawings are in black and white, but mostly visible. Only a few are difficult to see. However, they are in 2-dimensions and they don't show you HOW to do the thing, they only show you the general area and the part you want to manipulate. The phrasing in the instructions can be hard to follow without a good visual reference unless you're already a master bike mechanic (which you aren't if you're thinking about buying this). So, buy this for general reference and rely on Youtube for video. The two together are better than either one alone.
The fabric attachment system is low-tech, but that seems to be helpful, as it can adapt to seat rails of different widths. One fancier option you might see on more expensive seat bags is a quick-release mount that you install under your saddle. But those have a fixed width, and therefore can fit under seats with only those exact specs. Brooks saddles, for example, are too wide for these mounts.
There are two major types of headset- older, threaded headsets that have a quill that drops into the stem of the fork creating an interference connection, and newer threadless headsets that the stem is bolted and clamped on to. Flat handle bars are preferred by off road cyclists who can ride in a more upright postion whilst still having the controls at their fingertips, whereas drop handlebars offer more cycling positions and are favoured by road cyclists who typically spend more time in the saddle. The controls attached to the different styles of bar are operated and maintained in different ways.