NY #9, President, and Bluebyrd are the European plum varieties that I had seen people having good experience with regarding black knot which had been your biggest priority. These were also the European plums that were indicated as black knot resistant by Purvis Nursery which also offers scions, so I had come across these in my research on the topic of black knot resistant European plums in the past Plums - Purvis Nursery and Orchard.
I had not seen much on the resistance of Early Italian but in researching that further I found that Lonster gave an update in 2024 Tumor/Gall on my pluot tree - #14 by Lonster that suggested that Superior and Alderman are his standouts for resistance (12 years without infection) and Maneta, Early Italian, Toka, and Au Rubrum are younger trees that he has that had avoided infection so far. In the end, it looks like his Bluebyrd and President had knots.
Given what I have seen, Early Italian, NY #9, President, and Bluebyrd are better than average for European plums in the area of black knot resistance but if your pressure is high enough there is a real risk of infection. Other things to consider are that NY #9 is probably a mild flavored plum that is better when dried (per Purvis under name Kenmore) but has good production and some rot resistance, President plum can have brittle wood ( Plum - President - tasting notes, identification, reviews) but is late season and some reports of good flavor, and Bluebyrd was tested in warmer hardiness zones so you are probably right to be concerned about hardiness but has had good reports on flavor.
I think the most resistant varieties like Superior and Alderman are getting the resistance from the American plum genetics and will give you the best results over the long term (note that not all hybrids will have the same amount of resistance but these two seem to have good reports from people who have grown them). The Wisconsin trial has some interesting notes on these varieties along with other hybrids that you were looking at. Japanese-American Hybrid Plums – Uncommon Fruit.