Definitely dont bother with buying a pi if you've got other hardware.

I have one physical (a 3b I had no use for anymore), and two running as containers. The containers do most of the heavy lifting, since they are so much faster than a pi they respond far faster, but the physical is nice for when I take down the clusters for maintenance (or when I lose power, the clusters shut down after about 3 minutes, the pi will keep going for a while on UPS).