As others noted, lettuce is not that good bang for the buck, and bread keeps reasonably well in the fridge. Oatmeal is great and can be left overnight in the fridge and be ready for the morning. I do mine with a tablespoon of flax seeds, freshly ground, whatever nuts I have lying around or peanut butter, frozen berries (a lot cheaper than fresh ones, our Kaufland has the ones I like, Lidl tends to add cherries but YMMV in Germany) and plant milk of course. Any seasonal fruit goes well in oatmeal otherwise.

If you don't mind spending 20 minutes on a stove you can also make a salty version with some soy sauce, paprika, onions and a little bit of (rice) vinegar.

Not sure how much it costs in Germany but a small block of tofu, imported from Germany, in the Bulgarian Lidl costs 1€. There are multiple varieties. Basil tofu goes well in salads to add some extra protein.

Again it involves a bit of cooking but lentils are cheap fast cooking protein. Red lentils with bulgur Turkish style are love.

Turkish stores probably have Cig Köfte in the fridge. It's made of bulgur wheat, spiced, very nutritious, and relatively cheap. Goes well in wraps with hummus and tomatoes/cucumbers. Falafel is great too but it might cost more.

Explore the cuisine of other countries, especially Middle Eastern, Indian, and ones that are serious about Christian fasts (Ethiopia, Georgia, Armenia). They use ingredients that don't cost much, the meals are delicious, and oftentimes it's easy to prepare.

As for boxes, I use old brine cheese boxes (don't judge, leftover from my vegetarian years) in a plastic bag to prevent leakage. Otherwise make sure it makes a tight seal.

Shop smart, in the veganDE lemmy community there is a weekly post about vegan discounts in Germany. I would kill to have that here, might as well do it myself someday.

Not sure if I am of any help but best of luck. Cycling to work builds up muscle. Your endurance can only get better with time. I'm seriously considering cycling rather than walking too.