Instead of the Purge meter having a cap, have a hidden counter that continues to build up even when your base is ready to be purged. Then have the purge go after bases that have the highest purge meter. So instead of purges being random, they hit the bases that haven’t had the purge in the longest of time.
P.S. I’m not sure if the purge meter grows faster the larger your base, but if it does then set a cap on how fast the purge meter fills; so large bases, or clans with a bunch of bases, don’t get favored for purge attacks.