One of the big things this game is about is adapting your job choices to beat each boss, and putting together a character from the strengths and abilities of each job. The fact you've stuck to one job for most characters is the main thing causing you difficulty here - you don't have the diversity in options to adapt to each boss fight or put together good teams. And later in the game, good teams are very different to good early game teams. Like, you might well not have any mages, and will be utilising two very different jobs on characters to get good effects out.

If you are adamant that you don't want to change jobs, well, there's not too much advice that I think I can give, I'm afraid. You probably at least want to switch jobs a little to pick up secondary job commands and abilities, however, even if you end up switching back before too long.

I would recommend that you change one or two characters jobs at a time. Jobs above level 9 get very difficult to level until chapter 5, due to the massive JP needed for them. With the JP it takes to get a job to level 11, you can get four jobs to level 9.

If you, say, changed Agnes to a Salve Maker with White Magic job command, she would still be a healer, and would still have access to that White Magic, but now she'd start going through a new, job set.

Similarly if you changed Ringabel to Time Mage, or Summoner - still offensive magic, but with new options. Red Mage also gives you a mixed offence/healing mage option.

For Tiz, you can go Pirate, Ninja or Swordmaster for more defensive physical options. Personally I wouldn't bother having a 'tank' on the team, since too many enemies target all and/or you have little control over who they target. You could adapt the role a little to a general 'support' role, which Salve Maker and Performer do excellently - Performer even kinda runs as a tank, since it can boost your whole teams P.Def.

For Edea, Valkrie, Spell Fencer or Monk would be good jobs to boost physical attack. Each offers something different, and keeps her role as physical attacker.