Hello @partigiano,
“none” flags when you are running a bridge is normal, you won’t get any flags unlike the tor middle relays.
To give you my experience about Raspberry Pi, I tried to run a middle relay on a raspberry pi 3B and given the number of connections and the bandwidth to manage, it was very often overloaded. The CPU was blocking at 100% which was causing problems. I have set limits of 2 MB/s but even so, from time to time you can have spikes in connections that make you overload. It depends on the usage. Again, this is a different case from yours.
For a bridge, this requires far fewer connections and bandwidth since connections are made based on demand and distribution mode (which distribution mode are you using ?). For me, the raspberry pi 2 will work without any problem if you are careful to set a bandwidth limit to avoid CPU overload. The bottleneck will be the CPU rather than the bandwidth in this case.
Each bridge is very important and necessary for the proper functioning of the tor network, your new bridge will help many people to bypass the censorship. Since the bridge is not used as much as a relay, this is the best solution for a Raspberry Pi 2.
This is my thoughts on your question, I’ll let someone else add their experience on the subject if necessary.
Thanks for running a bridge for the Tor network 