the bitterness comes from the white layer that’s between the peel/skin and the “meat”. if you remove that layer. this problem will be solved. just give it another chance, i believe you won’t regret it. the long boiling is mainly for removing such bitterness(Naringin) of that layer. [btw, fyi, there's a Chinese/Cantonese dish called 柚皮(i don't know what it called in English, maybe Pomelo peel, usually served during the mid-autumn festival), its preparation requires the white layer be forked through and boiled for 5 minutes in water, and let sitted in the liquid for 1 hour, then the liquid removed(remember to squeeze dry the layer) and fresh water be added in to soak for another hour, and repeat the procedure for 4 more times (the repeated soaking and rinsing removes all the bitterness). that white layer is ready to cook and eat then.]
whenever i made this cake, i used fresh orange–ground the peel(only the orange part) and mashed it with sugar to free all the orange essential oil within, it smelled heavenly. then removed the white layer(most of it usually, i just peeled it with my fingers, easy), and put the meat in the blender to make it a mush. this way, zero bitterness!
also i replaced 1/7 of the almond with shredded coconut(coconut meat) and another 1/7 of almond with plain flour, this gives a better volume to the cake. and to enhance fluffiness, i separated the egg white and whipped it to a standing foam before mixing it with the main batch.
sometimes, i added 1/4 small lemon peel and lemon juice or added a pinch of cinnemon to play with my tastebuds.
i will love this moist cake always.

P.S. to strengthen the almond flavour, i use sliced almond instead of almond meal. brown the sliced almond in the oven or pan over stove, then put the air cooled brown sliced almond in a plastic bag, grind them to meal form with a rolling pin or wine bottle.

You blend everything to a fine consistency.

Thank you. Grace