Another excellent manual on gravel road maintenance is A Ditch in Time.  There is a link to it on the Resources page. 

One recommendation I have heard is to figure how many years you can go between needing to add more gravel, divide the road into that many sections, and add gravel to one of those sections each year.  When you get to the end, start over.  Or, if you can afford to do more than that fraction of the road each year, do what you can afford, and then when you reach the end you can have a gap before you need to start over.

I've never heard of Perma-zyme, but it sounds intriguing - if it actually does what it says.  So I went online looking for reviews, and found the following.  If you go down the thread, there are mixed reviews, including a rather in-depth study by someone wondering if it could be used as an alternative building material, and someone else who was trying it for a road.  So far the thread doesn't have any review from anyone who used it years ago, telling whether it held up or not.  Anyway, you might want to read through this and draw your own conclusions.  If anyone else has actually used it, I'd love to her how it has held up.

  https://permies.com/t/161414/DIY-Permazyme-recipe-alternative 

I did finally find one success story, but it's from Michigan, and their soil type and weather conditions may not be the same as ours.

https://www.farmshow.com/a_article.php?aid=36705

At one time there was a study done (based on the "Fourth Power Law") that concluded the impact of one five-axle tractor/semi-trailer on a road is equivalent to 9,600 passenger cars.  I remember when it was reported in AAA magazine decades ago, and it was accepted widely for some time.  Having seen what a logging operation had done to our road, I readily accepted it as fact.

On the other hand, a more recent study that was based on actually running trucks repeatedly over a stretch of pavement arrived at a much lower figure - somewhere around one truck to 300 to 350 cars.  But that was based on paved roads, and it was noted that the thinner the pavement, the higher the damage.  Obviously, the effect on a gravel road will be much more pronounced - but probably not as high as one truck to 9,600 cars, much as I'd like to think so!   Still, even at a ratio of 1 to 350, that means that one single round trip pass with a heavy truck can take a year's worth of once daily round trip passes with a passenger car out of a paved road, and more out of a gravel road.  I'll attach the study that explains it all. 

I completely agree that heavy trucks do a great deal of damage to roads.  I do think that the figures from the study I cited are not the whole story for gravel roads.  The study even says that the thinner the pavement, the higher the damage - so obviously, if there is no pavement, there will be a lot more damage.  But I also know that when you try to claim 10,000 trucks to one car, people who drive heavy trucks will vehemently disagree. Some will even try to tell you that because they have more tires to distribute the weight, they actually do less damage than a car.  In answer, I would simply show them a picture of my husband trying to get out of what was left of the road to our house after it was used by a logging operation.  (See first picture.) To be fair, this road had NO gravel, and this was in the 1980's.  The industry has since cracked down on such abuse to a large extent, but even if they use a road only when it's dry and firm, the sheer weight of the trucks compresses the road surface in their two tire tracks, resulting in destroying the crown so rain water can't run off the road.  (See second picture.)