The most common cause of bitterness in coffee as I understand it is over extraction - which means not enough fresh grind in the basket for the volume of coffee being extracted in the cup
I use 22g baskets at my work but I still will not extract more than 50mls max from any double shot, as I don't like the flavours that the coffee produces when extracted past this point, and in most instances ill only extract 45mls for a double shot (read long mac / double espresso) and would only extract slightly longer for a long black or a 12oz takeaway)
I'm guessing places that use 7 or 14g baskets are extracting their coffee a lot longer than that, and as a personal taste preference I either find the coffee too weak and / or over extracted for my liking
Grinding to order is also a big issue here coz unless the ground beans are used within a minute max of grinding your going to start losing flavour, and in my expedience at least this is still an issue for many places
For a long mac I start by preheating my glass, by half filling it with boiling water and leaving it in their for 30sec before dumping, then running 45ml of coffee from a triple basket, topped with a layer of textured milk, and a drizzle of cold milk - so that no more than half the latte glass is filled in total volume
Others may have different ways of macking a long mac, but this is what works for me, as the resulting flavours are smooth and velvety, sweet and strong, without any overt bitterness
I think the milk that you're using makes a great deal of difference as well. Folks have different preferences but for me it's unhomoginised all way for super creamy texture and flavours which meld effortlessly with the coffee
Good luck on your coffee quest!