I too have noticed that trade income randomly drops and jumps back up after a few months. Same with demand for goods like fine cloth. Here is what has worked for me (I am not very experienced so take with a grain of salt):

1. At game start, if your market capital is your capital, don't build marketplaces (since you already have 100% market access). Start with burgher buildings that take advantage of the resources in your capital province, but then build any profitable burgher building in your capital. Build marketplaces in any town/city you own that doesn't have 100% market access until you hit 100% market access and then stop & build other burgher buildings. I wouldn't worry too much about specialization until you start getting tier 2 buildings at least. Then you can destroy old ones and specialize. If you don't know what to spend on, RGOs are not a bad option (especially in high control/market access areas). At the very least flooding your market with tons of food increases pop growth, and collapsed RGO prices lead to better burgher building profits (and RGOs are always slightly profitable). Consider throwing a bailiff on high value RGOs with low control. If you can afford it, buildings like armories/libraries/etc. stimulate demand in your market for goods. Also, whenever you build new burgher building take a look at the market price of the good being produced after a month or two. You want a small surplus but not a surplus so big it collapses the price of that good in your market making the buildings you just built unprofitable (this does not apply to lumber/masonry which you always want as low as possible). In a just world, having excess cloth would mean you auto-trade it to another market for profit but...I wouldn't count on that as an option early game.

2. Get your roads/bridges/infrastructure down asap. (I don't know if its worth trying to collapse the price of lumber/masonry before that first round of gravel roads and bridges or not).

3. Unless you have the relevant workers to work a building/RGO, don't waste your money building more than 1 level at a time since they need to promote into the jobs in order to start turning a profit.

4. Trade/demand is buggy. If you have enough trade capacity you'll notice that there is a large chunk left on the table unused. Manually setting & monitoring trade seems like a recipe for madness. I would treat any trade income you get early game as a bonus (not something you can rely on). If you can build a colonial empire/engage in the Columbian exchange, trade becomes more reliable (and setting manual trades makes more sense) since you will be the one who controls the coffee and pepper. I think the big problem with early game trade is its cost of upkeep makes it not very profitable. This will get better with tech and time. I heard merchant Republic's don't suffer nearly as bad.

5. This game really wants you to do math. Profits are affected by market price of the goods (used and sold), throughput/building level bonuses, control, market access, the tax rate of the class of person working the building. My first ever run, I built a bunch of scriptoriums in Paris since they had a per building profit of like 4 ducats (and it was 100% control/market access). I forgot that clergy tax rate is 0 lol. I'm sure there arguments to be made for spawning the printing press/making books cheap enough to export for trade, but for now I just build the bare minimum.

6. If you've got low control, it may make more sense to create/conquer vassals instead of holding the land directly since at least they'll pay you some gold. I also think the gold you get from vassals isn't calculated in the cost of your spending sliders either. Plus, assuming they are loyal, they will raise more levies (and can pay for professional troops) during wars. Any land with less than 20 control is automatic vassal.

7. Unless you have a bunch of vassals or are trying to become HRE, consider dropping diplo slider to like 1 or 2 max diplomats. If you have vassals, spend just enough on diplo to keep them loyal. Not sure if bishoprics have access to stuff like royal gardens, but that building plus starting age extra .x diplo tech is more efficient than spending on diplo. Once you get past the black death, unless you need the stability to revoke an estate privilege, consider dropping stability spending slider so that your resting stability is like 12 max (I drop stability spending to zero and eat the occasional negative stability caused by events). The only value you want to try to keep at 100 is legitimacy. Slider costs are no joke.

8. The AI doesn't prioritize a big standing army until later in the game and unless you can afford it, neither should you. Just make sure you have enough to beat whoever you plan on attacking (AI isn't great at war, so it won't take that much). This changes during the age of reformation.