I milked 2 jerseys, a guernsey and a holstein x jersey and loved the milk of each one of them for different reasons. the jerseys gave more butterfat and the butter was excellent

one jersey kept the fat for her calf (yes, cows can keep the fat for the calves and give you skimmed milk- which had amazing selling appeal) the guernsey gave lovely golden milk, which appealed to some people. the holstein x gave lovely white milk which had appeal to some people. in TN it is illegal to sell milk off the farm so i sold cow shares. some would state the breed of cow they wanted milk from. the established breeds are good enough for the niche dairy. you don't have to reinvent the cow.
it sounds like you already have a good body of knowledge so i will say that i learned i liked the smaller jersey and guernseys to the larger holstein or swiss. working alone, i just like the smaller size better for handling, vetting, freshening, shelter, etc. i also liked their docile temperaments. i left the calves on them; didn't have a lot of customers to start. so the larger sized cows give more milk but seems like they give more milk because they are larger and they eat more. i've read the jersey actually gives the most milk based on body size. but dunno if that is correct.
it takes a while to get the customer base going. i didn't hang in there for the good customers to add up- i worried about liability with raw milk. people would come without a cooler... sometimes their jars didn't look sparkling clean and i would wash them even though i had stated i wouldn't. some but to clabber the milk and i worried about that.
look into a small, expandable line system from the start if you can afford it. the buckets won't work out for three cows that don't have calves on them. you'll have to stop milking, dump the milk in a cooling tank, clean bucket, and start milking again.
being small has great advantages too. you can take a personal approach to the cows, keep them clean and healthy and producing a healthier product. you don't have the factory line mentality of speed, speed, speed and sucking the life out of your cows for extra pennies. it is a lovely way of life. the 7 day a week schedule didn't bother me because i loved what i was doing but it was expensive and i couldn't hold the line until breaking even, which i didn't do. think it would have taken about 5- 7 years to develop the customer base. in my neck of the woods people didn't want to pay $12/ gallon. I got a little over what store bought milk puss sold for and butter was not worth it. yet my cows were vetted and tested, fed well and treated well so it was a losing proposition. if i had to do over, which i still dream of, i wouldn't sell for less than $10/ gal. i would rather raise pigs than sell for less. the small farmer has to stand beside her superior product and expect to receive its worth on its own merit and not competing against super walmart prices. there is a demand for excellence - it is just a long timeline. best wishes for you.