The ratios aren't spaced out consistently
Actually, if you look at your diagram and exclude the smallest and largest cog on the cassette, the gears are more or less evenly spaced. What unevenness there is is an artefact of the requirement for whole-number ratios. Sure, replacing the 15t cog with a 14.5t cog would give better spacing between the 17t and 13t but there's no such thing as a 14.5t cog.
Typically, you wouldn't use the smallest cog, except with the large chain ring, or the largest cog except for with the small chain ring. This is known as cross-chaining and it puts the chain at an awkward angle where it tends to rub against the front derailleur and is generally inefficient, and you'd be better off using a different chain ring.
To go down through all the gears from highest to lowest, you will find yourself switching front gears several times back and forth while simultaneously switching the rear.
That's not how you use derailleur gears. Rather than thinking in terms of a sequence of 21 gears, you should think of three sequences of seven. Any time you run out of gears in one set, you move to the next chain ring and adjust the rear gear by a couple to compensate. So, in practice, shifting up through the gears would look something like (read left-to-right, top-to-bottom, as usual)
28:28 28:24 28:20 28:17 28:15
38:17 38:15 38:13
48:15 48:13 48:12.
Note that, unless you shift front and rear simultaneously, getting from 28:15 to 38:15 will involve spending a moment in 38:15 or 28:17. I'd make both shifts while turning the pedals without putting any power through them so it doesn't make much difference what order the shifts happen in.
Shifting back down again would probably look something like
48:12 48:13 48:15 48:17 48:20
38:17 38:20 38:24
28:20 28:24 28:28.
Notice that only about half of the possible combinations get used in each shift pattern. By the way, you wouldn't sit down and learn these sequences: they're just what happens when you change at the back where possible, and change at the front only when you need to.
In practice, if I was accelerating from a stop, I'd be much more likely to just shift one derailleur at a time and do something like
38:20 38:17 38:15 (accelerating fairly hard)
43:15 48:13 48:12 (taking it easier; the first of
these is a big jump in ratio)
Many of these gears are so close as to be redundant.
This is the same misunderstanding as above.