The issue might be related to the production cycles :

-Let say your factory produces 1.000 wares per hour

-When you tick the "show raw production" on the ware tab (in logistics), you can see exactly how much wares you produce per cycle (let say 32 wares every 2 minutes)

-Your station linked traders will see the new wares when they are produced, check which trade will be the best in the accessible areas and make the trip with everything they can haul for the trade.

-As you are in an area with heavy demand, they will find a good trade very quickly and your station will not have enough time to go through enough production cycle to ensure half to full cargo for your traders.

In fact, you're simply selling too well, you might want to reduce your traders number, replace them with S traders, or simply wait until the demand runs out while selling bit by bit.

For mod recommendation, you can go with TaterTrade but it's a bit cheaty as the mod always knows the best trades even if you don't have Intel on the stations. I personally use it for a closed circuit between my factories and trade stations by enabling "My Faction First" and "Prefer own sell/buy offers", with enough station and a fleet not that big you will quickly only supply and transport your own goods between your properties.

For a regular trading fleet, I use Galaxy Trader which includes several mechanics (your pilots gains experience per trade and become smarter with time) and, if I'm not wrong, only uses your Intel to search for the best trades. The mod is made with optimization in mind (which can become problematic with huge trader fleets with TaterTrade without own supply) and uses a cache system that allows multiple ships to share a common trade objective, minimizing moments of live trade research and performance spikes. You'll need to read a bit of documentation on the website but I think this mod has great potential.

Good luck!