What factors to consider for a long lasting salad?
-Make slaws of hardier vegetables - bell peppers, carrots, unripe papaya, hard cabbages. Both kinds - the ones that are slightly cooked with a boiling vinaigrette, or the ones prepared raw (som tam for example) , can last a few days in the fridge.

-Pickles tend to be a hardy ingredient.

-Keep a cold but not freezing fridge (I recently started a discussion here ... Why do fridge temperature standards between US and other countries differ?  read what EVERYONE wrote )

-If there are leafy greens involved, either store dressings separately, or only put the acid and/or oil on the salad before storing but nothing that brings in salt or sugar. The acid could help, esp when there are fruit in the salad, but could also cause problems...

-Look at plant based proteins - TVP, tofu (optionally smoked), burmese tofu, tempeh, seitan... fried and maybe spice rubbed and/or marinated and/or breaded... cooked beans and lentils... less risk of meat-related dangers... keep fried items in separate container so they do not get waterlogged

-Potato and pasta salads could work for you...