I ended up using speed indicator together with cadence sensor. You can remember which cadence it should be at some speed or which speed it should be under your usual cadence. This is gear specific.
This allows to differentiate quite easily between the three fastest sprockets I mostly use.
Counting clicks also works in general but I tend to lose the count, unless "two up" - this short but steep uphill - "two down".
When the smallest sprocket is selected, the "faster gear" lever can move larger distance before touching something inside the mechanism. This allows to differentiate between smallest sprocket and others.
While I was initially convinced that the rear gear cannot be visually seen by riding cyclist, @Adam Rice is right: yes, it is visible when looking down and backwards between the legs. Just do not look there for too long as while doing so you do not see forwards.
On an ordinary bicycle you actually do not need the gear indicator much. You pedal with cadence you prefer. If it is too difficult / unnecessarily easy, shift the gear up or down. But on electric bicycle wrong gear often does not feel very wrong immediately if you crank full assist. It comes later as an early component wear.