i’ve tried that too, but it turns out my thumb is horribly poorly calibrated. fortunately, a battery-powered pressure gauge runs forever on a coin cell and really doesn’t cost that much. (i’ve had bad luck with mechanical ones, they haven’t lasted very well for me. also they don’t have nearly the read-out range of an electric one.)

If you pay attention to the amount of squeaking that happens when you turn on smooth pavement, that is also a very handy giveaway for what kind of pressure is in your tires.

occasionally squeezing it by hand has worked fine for me so far

Even if you wanted pressure monitoring I can’t help but wonder if there’s a cheap mechanical way to do it. Something like a green/red indicator that flips at 20% under your desired pressure.

Oh, you still have to take the pump with you. But at least in Europe they are way cheaper; 70€ new and as low as 30€ second hand a pair.

Quite handy on tour to get the tire back to the exact pressure you want without guessing and hopping on and off the bike for multiple attempts to get it right.