I've only done a small amount of vegan pastry, so I don't know if I'll be much help. Mine have been mostly cakes, buttercreams and mousse type things.
So for egg, there is the flax mixed with water as a sub, in cakes it's not much of an issue, because usually a plant milk is mixed with vinegar to react with the baking soda to help with lift.
I personally haven't tried the Bob's Red Mill egg replacer or the Just egg liquid egg yet. Another egg replacer is aquafaba (chick pea liquid from the can). Those might be something to experiment with. I've heard that the Miyoko's European style butter is nice for baking, but haven't tried it yet. I made vegan butter before and it had a nice flavor. I used it for a vegan wedding cake.
You could look into some of the vegan yogurts, sour creams or cream cheese that might have more of a tang like buttermilk, could be thinned with plant milk to get the liquid consistency.
Keep in mind too that you should look for vegan sugars, no bone char for processing.
