Overkill in wire gauge is a good thing for us most of the time. You don't have to worry if your signal is ok. (there are some frequency losses possible with some wire, but folks seem to manage fine)
Gauge enables distance with less voltage drop and adds amp delivery... your loss of power really likely boils down to mediocre power on tap. More amps/watts would have not likely not needed the drop moved.
This suggests something has a weak connection still. Hopefully; like tubular track; connections improve with some run time. More drop wire is really needed also imo.
16g is fine for that size layout. The Z1k may like 14g even better.
Over gauge wire is safer from meltdown if/when things go wrong. Meltdowns of wire looms = 🔥🚒
I'd say your next step is a better power supply and after that, more power drops to the track (did you get the Z1k?)
Then you choose wire by the new transformer max amp output per ft of wire. (Add a gauge if close to chart max..search AWG charts X-amp X-ft. at 25v or more. These are mininums for saftey, not a max ) Then choose external fuse breaker aimed at max amp expected to be used. (close match external thermal-breaker or SloBlo fused for modern power; or looser matched, over the expected amp a bit, use magnetic adj. external breaker #91 Lionel, similar, etc or normal fuse on old transformers)
4 trains is a bit hectic on "derailment day".....those days when flange gremlins get bold.
I have four lines on a similar size table but usually run three and mess around or tinker on the least populated or nearest of the lines just for convenience. I like the dogbone vs fig8. ... but I did that too so I'm biased 😁
Maybe with #4 being a trolley? A great "always on accessory" imo. 3 actual trains to control, 30-40 cars total is about right for me on a table. A trolley or two and fewer actual trains runnings during tinker time is much less intense to have to monitor.
I also like the concept of a small elevated line with a small engine and crane car, hopper/ gondola, maybe a passenger coach, but not much more. It will be servicing the businesses below by loading directly from yards onto the main line cars, visa versa &/or scooping to hoppers to be moved and dumped back to ground lines or bins/yards; maybe scooting a few local folk around too.