people would pronounce the a rather obviously over here
May I ask where you live?
Because to me personally Isolde and Isolda are quite different (my first language is German).
ee-soll-duh. (Isolda)
ee-soll-de (the de a bit like the e in egg...not really, But it's closer to the "egg e" than anything else, I think).
Whereas Isold... Idk, Wouldn't Ysolt be more obvious for the pronounciation you seem to want? I may be wrong, probably depends on where you live...