if there are not enough pool members in a pool send the traffic to another pool

Oh... I had to read that a couple of times more to understand that 🙂

In that case you could create individual monitors for each pool member. Assign each poolmember of prio group 1 five individual monitors (for itself and the four other prio 1 server) and have an up requirement of 4.
If only 3 poolmembers of prio 1 are online, this will mark all 5 as down.
Makes sense? Or a picture will explain more than 1000 words?

However, the above described solution doesn't scale. The iRule solution is far better in this case.