Bring 1 cup cream, milk, 1/4 cup sugar, and vanilla beans seeds in a small saucepan over medium heat to a boil. Whisk egg yolks in a medium bowl. Gradually whisk half of the hot cream into yolks until combined. Whisk yolk-cream mixture back into saucepan. Cook and whisk until mixture starts to thicken and steam (do not boil). Strain mixture into a medium bowl. Cover and chill overnight.
Heat a large nonstick skillet over medium heat; stir in strawberries, rhubarb, 1/2 cup sugar and water. Cook for about 10 minutes, stirring frequently, until fruit is softened and mixture is thickened. Puree mixture in a food processor.
In a large mixing bowl, using a mixer on high speed, beat together cream, confectioners' sugar and lemon zest until stiff peaks form.
To make ice cream: process vanilla custard mixture in an ice cream machine until frozen. Fold in lemon whipped cream. Fold in strawberry-rhubarb mixture and angel food cake. Place in a freezer-proof container and freeze until solid.