The trick is - check the seat map and then once you're on-board find the empty rows in the middle that are unfilled, and take the whole row for yourself

I was offered $99-$149 upgrades on BCN SFO a few times and took them up on that offer.
Prem-Econ is more like US domestic F, except the seat may be a bit narrower, nicer than DY's prem economy.
The food ex-BCN is decent, though nothing to write home about (DY has better food in prem. econ.)

Neither Level, nor DY are super strict with carry-ons if you bypass checkin. The standard 1+1 is allowed.