How to increase the perceived sweetness of homemade lemonade without sweeteners?
Lemonade is of course all about balancing the sweet and the sour.  It stands to reason that if you're trying to amp up the sweetness, you can either add more sugar (the opposite of the goal here) or reduce its opposite, the sour.  

Reducing both sugar and the acid is equivalent to diluting your lemonade, so one of the first things you could try is simply adding more water.  However, that will give you a more diluted and less flavorful end product overall, so let's assume that you're trying to avoid that as well.

With that as the goal, here are two things I would try:


Your method as written involves simmering the citrus peels in water. It's a good notion to extract as much flavor as you can from the peels, but water isn't the best means to extract that flavor; it's actually the sugar.  The reason is that many of the flavorful essential oils aren't water-soluble, at least not on their own.  Instead, try placing the peels in a non-reactive bowl and pouring the sugar over them.  Muddle (i.e. gently crush) the peels to break their cells and release some of the oils.  Then let the sugar stand and absorb some of those oils for 30-45 minutes.  This is known in geeky cocktail circles as creating an "oleo-saccharum" and is an essential step in making really good, authentic punch, the goal being to get as much citrus flavor as possible.  After the resting period,  add the sugar and peels into your boiling water, stir until the sugar dissolves, and then strain out the peels from the finished syrup.  With this method, you will get more citrus flavor overall, so you could dial back the juice slightly to taste in order to get a sweeter end result with the same citrusy profile.
This sounds counter-intuitive, but try adding a very small amount of salt to the lemonade.  I would say about a small pinch to 1/8 tsp in a full 1-liter batch, but you'll have to tweak for your preference.  With such a small amount, you won't actually taste the salt, but you will notice its effect.  Salt has a suppressive perceptual effect on both sourness and bitterness, and adding a small amount to a solution that is both sweet and sour (like lemonade) will dial down the perception of sourness.  Here's a previous answer that I wrote on this same effect related to suppressing bitterness, but the effect is similar.  Here's a chart that provides a great summary; the second line from the left shows the perception of sweet vs. sour, and the fourth shows what happens when you add salt.  Notice that the perception of sweetness stays nearly the same, while the perception of sourness lowers significantly.  It's a neat trick!


There are probably more exotic and interesting methods, but these are where I'd start.