How to reduce acidity (and increase strength) of moka pot coffee?

Mokas are designed for a fixed water / coffee ratio, given by the volumes of the respective chambers. As a baseline, you should always fill up both, for the water until just below the valve or to the given fill line (if your pot has it), the grounds chamber gets filled up to the top, but not tamped or compacted (or only ever so slightly).

That leaves mostly grind size and to a far lesser degree brewing time as parameters. Aim for a comparatively fine grind - finer than for pour-over, a bit coarser than for espresso. Try to go down with the grind size until you start to see some dust at the bottom of the upper chamber, but not so fine that it clogs up. As you have a largely fixed and rather short extraction time, coarse ground coffee won’t be extracted well, giving you an under extracted brew.

Some coffees, especially lighter roasts that have a more acidic and floral notes, are not ideal for a Moka. But assuming that you are using the same coffee as in your other post, a full chamber should be a good first step, the half-full chamber means you don’t get the desired pressure in the setup and an underextracted (-> weak and acidic) brew.

Grind fine as you can. Use little less water. Brew for longer. Don’t boil water, maybe try & bring temperature down using a food/meat/milk thermometer probe.Experiment with all these in small increments & failing that try a darker roasted coffee. Kenya & Tanzania coffees tend to be most acidic especially when lightly roasted.