Apart from my IP, connection times and amount of traffic sent, what other information does a guard relay directly know on me?
If my understanding is correct, due to the onion encryption it can’t know anything else right? certainly not the destination IP the exit relay connects to. I am not talking about correlation attacks and the likes, just the info exposed to that specific relay.
And, is there anything different in this respect if I use a Tor bridge?
About you? Knowing your IP is all that really matters.
Tor bridges function in the same respects as Tor Entry guards. Meaning they cannot see your exit relay or exit traffic.
Tor Bridges are used for censorship circumvention. If it is not considered dangerous or suspicious to use Tor in your location there is not much benefit if any from using a bridge.
For the majority of Whonix users, connecting to Tor with the default configuration is appropriate and will work successfully. The minority of users requiring a bridge normally fall into three categories: 
Tor is blocked, and some way - any way - to reach the network has to be found. The adversary is not very dangerous, but very annoying.
Tor may or may not be blocked, but the user is trying to hide the fact they’re using Tor. The adversary may be extremely dangerous.
Other bridge users: Testing whether the bridge works (automated or manual), probing, people using bridges without their knowledge because they came pre-configured in their bundle.
Not sure I understand your answer here. What good is an IP by itself without knowing nothing about the activity and assuming no correlation / cooperation with others involved is done? isn’t that the whole point of Tor? that no single malicious relay can by itself be a problem?