Synapse is a common server because it is the most featureful, but other servers like conduit and dendrite work fine... Bridges are written to interact with the feature set of the server, so other matrix servers may not support all the same things that make the bridge work.
Not sure about the yunohost setup, but there is a lot of documentation around the Ansible deployment and it includes every bridge imaginable, I would start there for a better supported solution.
moving to AGPL is "less permissive" than the apache license they currently use? no. this is just plain wrong. the supposedly less-permissive part is the CLA they are asking contributors to sign so they can dual-license the software in some situations, but the CLA isn't even written yet and they are actively listening to feedback from the community to determine how best to shape this license agreement so that all parties are happy.
this is FUD.