I have been growing Methely for 25 years. Here in S. NY it is a major black knot magnet, although I’m not sure if native cherries are part of the mix of trees on LI, which is a key host for the infectious fungus.
As far as flavor, it is below average for a J. plum as is Shiro, which is not to say it isn’t delicious when fully ripened after being zealously thinned, but thinning can be a PIA because it often sets its fruits in tight clusters up and down the branch work.
Shiro would be a better base tree, although I’d start with Satsuma just because it is a much better plum although its drooping branches and greater susceptibility to leaf spot and maybe other issues I haven’t identified could create problems, but if you grafted other varieties on it they wouldn’t be affected by that. Ruby Queen would also be a very good base tree and may even have resistance to black knot. It isn’t as sweet as Satsuma but I find it delicious as do most I’ve offered it to. It is my Elephant Heart replacement which is the world’s best flavored Jap plum when it is right but it has real issues here.
The only verified black knot resistant varieties I grow, besides an experimental, unreleased Cornell variety, are European, with Bluebyrd seeming to be that and, to a lesser degree, Italian.
In my nursery I am going to begin favoring Bluebyrd as a stock tree for multi-graft plums because it is a strong grower but also fairly early bearer for a Euro and from a small sample I’d say the fruit is delicious. Of course, all comments about flavor here are based primarily on my individual tastes.
Once you learn to do it at the right time, grafting plums is pretty easy on vigorously growing trees in good light. We could steer you through that, but you shouldn’t attempt it until your base tree has been growing for a season.
