Prepare the strawberries by combining sliced berries with granulated sugar and lemon juice in a bowl. Stir gently and let sit for 15-20 minutes until juices release and berries soften slightly.
Cut the cake into 1-inch cubes while the strawberries macerate. If using pound cake, you can lightly toast the cubes in a 350°F oven for 5 minutes for extra texture and flavor resistance to sogginess.
Whip the heavy cream in a large bowl using a hand mixer on medium-high speed. When soft peaks begin to form, add powdered sugar and vanilla extract, then continue beating until medium peaks form (cream should hold its shape but still have a soft, billowy texture).
Begin assembly by placing a layer of cake cubes at the bottom of each serving glass, pressing down slightly but not compacting them too firmly.
Spoon a layer of macerated strawberries with their juices over the cake layer, allowing the juices to soak into the cake.
Add a generous dollop of whipped cream on top of the strawberry layer, spreading it to create an even layer that reaches the edges of the glass.
Repeat the layers once more: cake cubes, strawberries (if using jam, swirl a small amount into this layer of berries), and finish with a decorative swirl of whipped cream on top.
Garnish each trifle with a whole strawberry, mint leaf, and/or lemon zest if desired.
Refrigerate for at least 2 hours or up to 24 hours before serving to allow the flavors to meld together perfectly.