Over the years, I’ve come across plenty of articles that claim to tell you how to build up your cycling toolkit for cheap. Somehow the word “cheap” stops recurring in the long list of “essential” bike repair bits and pieces. The truth is, there is no real way to build a whole shop of tools all at once without plopping down a few hundred bucks. But in an attempt to show everyone that not all of us truly need a full complement of bike tools, I present the truest, most budget-conscious, and barest of bones options to you.
Spare tube:  Sometimes, a tube is just too damaged to be patched, or has had a failure such as the valve stem breaking off which isn’t something that can be patched.   To save weight and space, I usually keep a lightweight version of the right size tube for my bike.   Since one of the first things I do with most bikes that I own is switch them over to heavy-duty thorn resistant tubes, I just take one of the originals and put that in the toolkit.

First, I researched. I looked at Amazon’s top-rated products and their user reviews. Then I consulted Bicycling magazine, Gear Junkie, Bike Radar, Outside, and the occasional bit by Lennard Zinn via Velonews. I also found some worthwhile discussions at Bike Forums. Then I spoke with four experts, Ramona Marks, Scott Karoly, Cari Z, and Alison Tetrick, riders from all across the spectrum, who tour, repair, and race.
The retractable tube is stored inside the body of the pump when not in use, and it can attach to either a Schrader or a Presta valve, the two types of valves you’ll see. Some pumps can accommodate only one type, making them less universal. Toggling between the two is simple, which can’t be said of other models with itty parts that require nerves of steel to handle on a good day, let alone while wearing gloves, crouching in the wind, oblivious to the sewer grate a few feet away.

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?
I just got a tune up on my bike and it rides so much better now! It takes half as much effort to petal, it is like a new bike. The guys who work there are so nice and always make you feel like your time is important. They don't overcharge or sell you things you don't need. I thought for sure I needed a new chain, but as it turns out, it's fine! He gave me some extra foam for my bike helmet, free of charge! I'll definitely come back here for all my bike needs!
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.
Modern bottom brackets use a sealed bearing type construction that keeps water and debris locked out, so they seldom go wrong. Maintenance is impossible so they need replacing when they do wear out. Older style cup bottom brackets need to be re-greased every now and again, and occasionally will need to have their bearings replaced. There are a lot more teeth on the chainring cogs than those on the rear cassette, so need changing less regularly. Cranks usually a solid one piece construction so there is not much to go wrong.