I have a manual shift car without a tachometer and honestly, I don’t miss it. The problem with shifting by the tach is that you have to know which torque curve the engine is on at the throttle opening you are using. A lot of people seem to think that the full throttle torque curve applies for all throttle openings.
You might notice that at 1000 rpm, there’s very little torque difference between 20% throttle and 100% throttle. If the acceleration levels out above a certain throttle opening, that’s your signal to downshift, however, if you aren’t accelerating, and can maintain speed with 10% or less throttle, running the engine at 1000 rpm is quite ok, in fact, downshifting would only make the engine less efficient.