Its because of the rules of blind typeing courses that says thumb should be used only for space? If you got such a courses more than a year before and already mastered blind typeing, than stop worrying. Just use thumb for `x`.
> I've tried micro-optimizing special cases like this before, but then I have to think as much about typing as what I'm typing.
It got better with practice. Like blind typeing. All you need is to train you fingers for they do it without conscious effort of thinking. Are you play on some musical instrument? Try it, it teaches how to train your fingers.
things that can't be typed w/o moving your fingers from the home row are really easy to type.
That's why some people (like me) switched to Dvorak.
Your sentence on Qwerty was 43 top, 21 middle and 12 bottom row keypresses.
On Dvorak, it's 24 top, 45 middle and just 7 bottom row characters, twice as good!
"ithout" is also entirely home-row, so you wouldn't need to bother with "w/o". The word demonstrates Dvorak's other feature nicely, hand alternation and outward-to-inwards flow:
without
,gkjsfk (Qwerty equivalent)
RLRRLLR