WifiEspNow VS ESP-Now

There are examples in the Arduino ESP32 distribution. It should work the same on ESP8266. Or try this library: https://github.com/yoursunny/WifiEspNow

There's some other good tutorials for it as well, https://randomnerdtutorials.com/?s=esp-now and https://github.com/HarringayMakerSpace/ESP-Now

For power, I'd recommend Li-Ion primary (non-rechargeable) cells, whatever size works best for you. Those have essentially zero internal leakage and a 5 year shelf-life. If you've done it all correctly, it's conceivable it could be 5 years before you need to change the batteries (on top of a 30' ladder). You'd need a small high-efficiency buck converter to drop the 6.8V batteries down to 3.3V for the ESP, with a separate switched 5V linear regulator (one with a SHUTDOWN pin) to power the servos when they need to be driven. The ESP will consume 16-20uA in Deep Sleep, and the buck converter should mostly be off, except for when the ESP wakes every 3.5 hours (maximum Deep Sleep interval). If you're already using ESP-NOW to talk to the vent ESPs, then you could do a 'vent changed' check every time it wakes, otherwise it's more efficient to go back to Sleep quickly and wait until 12-24 hours has elapsed before you fire up a full WiFi connection. Not counting buck inefficiencies and servo movements, the ESP would only consume about 175mAh per year. The batteries will expire before they empty, maybe in 6-7 years.

I presume you've seen Anthony Elder's examples? He got a single ESP to act as hub by restarting it. In a low-traffic environment that'd work. There was discussion and a new PR to speed up the reconnect time using WIFI_RESUME that's been merged in the git repo, which should make a single-ESP hub faster to respond. You'd need a little fault checking in case the BSSID changes of course, but for the normal case things would be faster.