Reserve Planning for Wooden Fence

It sounds like your Association is being very budget conscious in maintaining the fence.
I congratulate you on this.

However, for planning purposes, you should consider the average life span of the fence based on the day it was initially installed.
A fencing company can assist you with this or some research on the internet should give you a good lifespan.

If, by your maintenance, you can expand the life of the fence, great!
But for planning purposes you should consider the cost of a full replacement.

You can add a separate line item in the reserves for post replacement (say x number for the lifespan) and picket replacement (x panels over the lifespan).
Your financial history should be able to give you this info.

Once the amount for a full replacement is met, you can replace or, simply stop contributing to that line item since the lifespan has expanded.

Yes, the posts can be a different line item in your reserve study. Our reserves specialist also has used other approaches. Instead of one RUL for everything, you can have, say, 3 RULs lives, say, 5 yrs, 10 years, 15 years expecting 1/3 would need something more or less at those intervals.

You can have two line items for the pickets one might be to repaint, and a different one to replace.

IF there is some aspect that must be done every year, let's say touch up painting, that line item would be in your operating budget.

As an entire chunk of common area that's the responsibility of the HOA it does need to be in your reserves study.

There are not expenses every year. A few years ago, there was a $6K expense. Last year, zero. It's almost impossible to predict what and when will be the next problem. But I am leaning toward you suggestion, and simply treating fence maintenance as an operating account budget item, perhaps at $6K every two or three years.