This will be an unpopular option, but I feel that we're very lucky to even have the Latitude + e-upgrade option to secure a J seat in advance.

Lucky because you a) need to have resources to purchase what amounts to an expensive Economy fare and b) have the e-ups to burn on the upgrade. The same is even more true for PY Flex + e-ups.

Do I know that I will confirm an e-up when I buy a LT / PYF fare? Sure, because I plan it that way. Do I 100% expect that I will fly in J on that flight? No. I usually do, but understand that could change, as it can for any e-up.

When I really, really want to fly a particular flight in J, I buy (cash or points) J. Though if I was 'bumped' in that scenario, I'd be really pissed too if the alternative wasn't palatable to me.