The biggest problem with your
The biggest problem with your system is that it doesn’t make full use of your batteries capacities, meaning you need to carry more batteries to supply your electrical requirements. Space may not be important to you in which case it doesn’t matter, but to a lot of vans it is at a premium. This is where systems that take into account the fact that standard voltage regualtion as fitted to 99.9% of vehicles only charges batteries to about 70% of their actual capacity.
100% charge can only be accomplished by more complicated charging systems. The one i have fitted i have fitted to 4 of my vans now over a period of 6 years and it has never failed me. These systems take into account factors such as voltage drop in your battery cable and temperature, both of which play a large factor in how much charge your batteries will take.
Also from my frequent trips to the alps in the winter months having a starter battery that is fully charged all the time rather than just one tghat is at best 70% makes a huge difference when you want to start your van in -15 and lower degrees. So your big chunky switch might not freeze but at the same time your battery might not have enough juice to start your battery. I have seen it happen many times and had to jump start several stranded vans.
KISS is fine but if everyone applied that thoery we’d still be living in the dark ages. But your system works for you so thats good.