How to use same set of modifiers for various shapes

Using the helper AnyShape type eraser

struct AnyShape: Shape {
    private let builder: (CGRect) -> Path

    init<S: Shape>(_ shape: S) {
        builder = { rect in
            let path = shape.path(in: rect)
            return path
        }
    }

    func path(in rect: CGRect) -> Path {
        return builder(rect)
    }
}

your function can be written as

func getShape(shape: Int, i: Int) -> some View {
    let selectedShape: AnyShape = {
        switch shape {
            case 0:
                return AnyShape(Rectangle())
            case 1:
                return AnyShape(Capsule())
            case 2:
                return AnyShape(Ellipse())
            default:
                return AnyShape(Rectangle())
        }
    }()
    return selectedShape
        .stroke(colors[Int(shapeColor)])
        .frame(width: CGFloat(shapeWidth), height: CGFloat(shapeHeight))
        .rotationEffect(Angle(degrees: Double(i) * Double(angleStep))))
}