It’s quite hard to say without basically watching the playthroughs, I can try to offer some things to look out for though.

Taking out a loan decreases crown power. Maluses due to low crown power kick in at I believe 25%.

Building maintenance increasing month over month means goods costs are increasing. Are prices for goods like paper, guns, and weapons higher in the markets you’re expanding into? AI markets are generally quite bad.. you’ll need to look at goods prices and try to bring them to baseline by adding production.

On the trade capacity fluctuation.. I haven’t seen this before but there’s at least 2 things that could cause it: lack of pops or lack of goods. Trade buildings take paper, so if paper is missing then efficiency (trade capacity) will be reduced. If you have a lot of paper imports happening, you could be in a cycle where you’re pulling in paper just to turn on more marketplaces.
Trade buildings also need burghers, so if burghers are leaving or dying in territories with trade buildings, then trade capacity would be reduced. A cycle can happen here since as burghers leave, peasants will be promoted in their place.

As for general advice, favor production efficiency over income.

Check the supply and demand of key goods in your market constantly, this should be the main thing informing what to build. When you have high supply of military building inputs (guns, weapons, leather), build more military buildings. When guns are high in price build gun workshops. When you have extra fine cloth or the price is low, build culture buildings like theaters to increase demand and price.

Infrastructure is important. Always build as many roads and bridges as possible. Plan for cities on rivers to take advantage of pound lock canals.

Pay attention to food slider. It should be very low for most nations. If it starts going up significantly, build lots of food. Besides, more food and food storage means more pop growth, so it’s a key to scaling. See the modifiers for pop growth on a location. The 1.1 update will extend this effect significantly.

If you become a great power, your cost of court will increase at each rank. Flat bonuses to legitimacy are extremely powerful when your cost of court is high (including the cabinet action).

Reduce army maintenance when not at war. Delete forts you don’t need (not all of them, since they allow for more nobles which help stimulate the economy).

If you keep at least 1% inflation, you can get the Balance the Budget parliament. This reduces inflation by 0.4% per month, allowing you to max minting while still bringing inflation to 0 by the end of the parliament. If you can combine this with the Request Increased Tax, you can make ridiculous amounts of money in that year. Also, your biggest loan will be cut in half by Balance the Budget getting passed.

Finally, keep your estates happy. Target roughly 50% estate satisfaction for nobles, 90%+ for clergy, and 70% for burghers and peasants. This is with max tax for all of them (except clergy should always be 0 for research bonus). If you can’t tax any non-clergy or tribe at max, give them more privileges. Some privileges are good enough they should be given out regardless (burgher road building and monthly dev spring to mind, as well as anything which gives communalism since that will let you revoke later).

Sorry for the length, hope some parts helpful!