Mix all the flours together in a deep bowl and set the mixture aside.
Heat the goat milk, it has to reach 37ºC (98ºF) so you can use the thermometer. Add honey and mix well. Now, add active yeast and psyllium powder. Leave it to create foam so that the yeast "eats" all the sugar. It will take approximately 10-15 minutes. Make sure the environment is warm.
Using an electric mixer, (wire whip) mix softened butter with the sweetener and salt. Mix for at least 8 minutes. Add orange and lemon zest, rum flavouring and vanilla extract.
At this point Add one by one egg at room temperature. Now add the milk and yeast mixture. Make sure the yeast created lots of bubbles. mix for another 8 minutes. Replace the mixer's wire whip with a dough hook.
Now start adding the flour mixture spoon by spoon. Mix constantly. When you use up all the flour, your dough might appear too thick. You can add 100 ml of warm water now, but it's not necessary.
Place your dough in an oven safe dish or bowl and cover it with parchment paper. Turn on your oven to 50ºC (122ºF). Place the dough in the oven and keep it there for 30 minutes. Now turn the oven off and leave the dough to rest for another 30 minutes. It should double in size.
Grease your hands with some coconut oil and work it with your hands for a couple of minutes. Divide it into 3 equal parts and create long strips. Place them on parchment paper.
Lay three strips side by side, pinching together at one end, and braid. Pinch together at the other end to hold the loaf intact. Return to the oven and let it sit for another 30 minutes. You can have the oven turned off.
Beat the egg yolk and wash your Tsoureki with it. Sprinkle with almond flakes and add some more egg yolk so that the flakes stick well to the bread. You can add some sweetener to the egg yolk, or maybe some cinnamon.
Bake the Tsoureki in the oven at 150ºC for 45 minutes. Make sure you don't open the oven in the process. When it's baked, turn the oven off and leave it in the oven for another 20 minutes.
Do not be tempted to cut the Tsoureki before it's totally cold. Ideally, make it on a Saturday before Easter, and serve it on Easter Sunday, like the Greeks traditionally do!