You also might try to utilize your family members ( FM ) efficiently, letting them govern your settlements with perhaps 1 additional "light infantry" unit ( like "Leves" = 120 men, low upkeep ), if needed, to boost up public order and profit from their management abilities, their positive influence effect, and, of course, their traits and ancillaries, wich can bring you lot´s of money. Example: your mining income would grow with a FM as governor in a while due to their "miner" = 10% and. later, "superior miner" = 20% ( so if a mine brings you 600 income, you´d get 660/720 instead; and imagine this with real rich mines upgraded to the second tier, with additional % of "Inventor" or/and "Mining Engineer" ancillaries ).
And do not pay attention at the "nominal" income of a settlement, instead take a look into the settlements details and see that it´s your armies, who those rich cities are paying maintance for, that´s why they seem to generate negative income.

Spain and Southern France are full of mines, wich explains the economical powerhouse Lusotania! They also control the whole trade on the coasts, so you might blockade their ports to cripple it and, at the same time, profit from those "rides" ( see it in your financial scroll once you start using piracy to your advantage :) ).
You can also use spies to incourage some unrest and force the AI to lower the taxes, and also keep track of enemy armies movements, so you can spot some pour defended towns and raid them with you troops, looting their cities and destroying the infrastructure. A plague is always a funny thing to cause some economical drawbacks on your foes.
You might retake some celtic settlements and give them back to the remaining gauls, forging an alliance with them, or try and involve Germans into a bloody war with Lusotanians.

The options are plenty, but it is a doom of the player, if one lets the AI really develop and steamroll it´s neighbors. Then you have to deal with one or two superpowers, like Lusos in your campaign definetly are, instead of several smaller factions, waisting their resources on fighting each other. Btw: it was a roman policy per exellance to devide and then counquer the weakened oponents, not to let a mighty empire on its boarders emerge and fear the extinction ;)

Good luck!