Are you still in Civil war?

In general, there are the following things: Raising taxes of provinces (I dont mean player owned fiefs, I mean taxes that governors send to the Emperor to something like 30-40%, I wouldnt set it higher. Later when your economy runs better, I would decrease taxrate (as high taxes cause several smaller problems)

Then higher personal to find tax loses, this increases tax efficiency (only recommended if you have high loses due to tax inefficiency)

Centralisation vs Decentralisation: Centralisation allows you to issue higher taxes, on the other hand increases Empire maintaince, while decentralisation does the opposite: lower taxes, but also lower maintaince.

Setting decentralisation to slightly decentralized is I think good, or even quite decentralized.

Governors send taxes to the Emperor, but this needs sometime, so it takes a while till you get money from your provinces.

If you own domus mare you can rent it out. (if you havent already, this is good as it doesnt cause any costs and the income from the rents usually equals the maintaince costs of it)

Investments in latifundia are usually something expensive, but rewarding for the long run (if you really invest a lot)

There is a quest given in Garama which gives you high reward if you choose the right options. (something like 200,000 denars) (if you havent done it already, this wont fix your economy of course but at least helps you to survive the time until taxes from provinces start to flow)

Once you have high debts there are emergency measures (letting the Roman nobility pay your debts) available when talking with the chamberlain, which will give you enough gold to repay all debts. (a hint there: if you dont manage and make debts, wait till you have really high debts before you do this emergency measures, because you will gain more money, but with the same negative effects). This options gives you something like 3/2 more money than your debts, so you will also have some money to invest afterwards.

There are other tyrannical measures to gain money from your settlements, but this is surely not enough to stabilize your economy. But if you own barbarian settlements which are not Romanized I would still regulary use the 'Extort subjects', then invest the gained money in workshops or latifundia. (of course ideally in Roman towns in stable regions)

If you own Egypt: make yourself owner of some of the villages there, or in general, make yourself owner of a village with a mine, ideally a gold/silver mine: This should be possible by removing existing governor when talking with minister. Mine villages produce a ♥♥♥♥ tone of money. (The villages around Thebae have gold/silver mines, also in the North-West of Spain are gold/silver mines, a village with a mine has a 'bandit lair' icon next to it, so you can recognize them straight from the map)

Other things to mention: Once you build a manor you can put slaves into your village. They will work there and produce much money, this is further increased if the village has a mine.

My approach in general is that I set long term preparations before becoming Emperor and that only happens after I secure funding.
So I come around Empire, do quests for cities and their lords, buy enterprices until I have one in every single city of the Empire and then focus on domus mare rents.
After that, I start to take latifudiae, this is very long term stuff, so usually Nero causes a lot of mess in the Empire I need to fix after becoming Emperor.
Once my monthly budget hits at least 200-300 thousands of denarii from enterprices, domus mare and latifudiae alone (they don't apply to tax ineffiniency as far as I noticed) I prepare to take over the Empire, preferably in rather smooth way to avoid civil war.
As for policies, I prefer maximized decentralisation plus aristocracy combination, sure, you get tax effiniency penalty but hey, you get 6 relations point per month with every commander and your Empire costs are rather low.
After becoming Italy I tend to prefer to secure all of Italy for myself as private domain rather than spread myself too far (much easier to manage that way).
Southern Italy is full of gold and silver mines villages, this plus manors with slaves and guards plus latifudiae alone gives you really high profit so you can sustain 20% taxes or even consider lowering them if you want.

Thank for the advice it really helped me understand why I had suck low income after becoming the emperor and how to fix it.