I wish I could give you an answer. I play on Normal rather than Hard, so I imagine that I am a less experienced player than you are. I tried playing as Carthage and found it harder than playing as a barbarian faction - Rome was coming after me and I didn't know how to respond. I love playing as the barbarian factions - mostly as the Iceni so far (I recently acquired the Caesar in Gaul expansion, so that I could play as more barbarian factions and to add more barbarian units).
Apologies in advance if I am telling you things which you probably know: playing as the Iceni, I focus fighting the war which I began with (at the start of the campaign) before fighting anyone else. You mentioned that you cannot make money. Are you using mercenaries? I try to rely on Levy Freemen and slingers initially. When I recruit mercenaries, I disband them as quickly as I can. Are you developing your infrastructure? I like building bronze forges (to recruit swordsmen) but I build a farm first - I upgrade this to the 'horse' type of second-level farm building, to recruit light cavalry. Are you specialising your regions (and, as you expand, your provinces) so that some have mainly economic buildings, instead of military ones?
Levy Freemen can be surprisingly effective: they move fairly fast, they have a couple of javelins (you can see Levy Freemen throwing javelins in the second picture of the Battle of Eborakon in my Iceni AAR,
here. They can be 'speed-bump' units, absorbing the enemy's charge (so that the enemy's best infantry charge is wasted on your cheap spearmen, while your best infantry charge theirs). Expert tournament player Decimus Maximus Meridius likes Levy Freemen so much that he named them the 'best unit in Rome II',
here - not the most powerful of course, but the most cost-effective. Obviously, some of his advice is more relevant to tournament players, but much of it applies to the campaign as well.
I wonder if anything I have learned from playing as the Iceni would help with your campaigns? I imagine that playing as the Galatians, Suebi and Boii would create different challenges and that different strategies would work. For example, playing as the Suebi, I imagine that you would want to take advantage of their Club Levy units. Other Celtic factions tend to rely on Levy Freemen (and similar units); Club Levy can be devastating against cheap spearmen. I have not played the Suebi, but I have fought them. Their archers can be very effective (I like their stealthy archers, who can fire without revealing their location) while their melee infantry seem to have lower morale than some other factions. So it seems that 'hit and run' tactics would work best for the Suebi, rather than the massed charge of swordsmen which seems like an obvious option for the Boii.