If you have to pour puddles big to get the mold to fill out, venting will cool the pour faster in the mold and make fill-out worse, yes, worse, and will not help fill-out. You may luck into something that helps you cure your thermodynamic problem, but a change in your casting method is the cure, not cutting vents in mold blocks. A good caster can cast any bullet mold with NO vents.
Big sprue puddles aren't a problem for me so your method is different, I just put them aside and use them in the next melt. That is not a sacrifice to me to get good bullets and most of my molds need that.
I haven't posted on swirl casting in a while but that is the most successful way to cure the zone cooling you have. You describe zone cooling on one side. Swirl casting swirls the metal rapidly in when you pour and counteracts cool zones of the mold during casting. Your turbulence pattern when you pour is the cause of your cold zone. Swirl casting also evenly warms the mold well when you do it and adds to the cure.
To swirl cast use no longer than a 1/4 to 1/2" stream from the pot nozzle or ladle, Tilt the mold 5 Degrees and flow your pour off center by half of the sprue gate hole diameter and into the high side. It is good to partially hit the high side of the funnel shaped gate hole Do not gurgle the metal into the hole, pour accurately off center into the high side.This immediately flows the metal into the mold swirling fast. The swirling corrects your other pour method that has caused zone cooling. Despite any inconvenience you have with big puddles, they serve a very important purpose. The puddle is a reservoir to flow in as cooling shrinks metal in the mold as it cools and the reservoir feeds to compensate. So the biggest puddle you can balance is a GOOD THING for good casting. The Lyman pressure casting method does a similar service by keeping the flow connected as the metal cools in the mold. But you can't swirl cast when pressure casting because of the tilt and off center pour needed to initiate swirl.
This is nothing new in casting and treatises were written in Hieroglyphics on swirl casting and casting reservoirs thousands of years ago by the Egyptian Pharaohs metal workers who were executed for miscasts when they failed with Pharaohs gold. I taught this stuff to Dental students and technical students as it also relates to metal flow in centrifugal casting in the Dental Lab.
Mold makers base the diameter of their sprue gate holes to make them 30-50% larger than the molten metal flow in. If your gate hole is smaller than that relation to your flow diameter, fix it. Air has to be able to get out as you pour metal in. Don't rely on vent holes to do that, pour better.