As you say, adding sugar merely adds another taste which "balances" the acidity, but doesn't actually reduce it. The only ways you can actually reduce acidity are to dilute it (which is not helpful), or to add an alkaline substance to counteract it by chemical reaction. But adding alkali to reduce acidity generally only works in very specific ways in cooking. The main alkali used in cooking is sodium bicarbonate, which will fizz, a technique used for raising cakes, but probably not helpful in fruit tart. The "lye" used to pickle herrings is also alkaline, but you don't want that.

My mother cooked with less sugar than most other people. You get used to it and will soon prefer it like that, like getting use to tea without sugar. I hate other people's over-sweetened (to my mind) gooseberries and blackcurrants.

Apple juice concentrate is of course high in sugar. It may be fruit sugar rather than sucrose, but it is still sugar. So thinking you are making something "low sugar" because you are using apple concentrate rather than cane/root sugar is not true.