The best way to migrate from synapse to dendrite will probably be by decentralised accounts. We have plans (driven by p2p) to let your account exist on multiple servers at the same time. The sort of way to do this would be to have @alice:example.com sign something on @alice:elsewhere.com to prove that she’s the same person. She then participates in all the same rooms from both accounts, which are presented as a single logical identity to everyone else.
So example.com could be synapse and elsewhere.com could be dendrite, and she would automagically replicate her account over between the two. If she turned off example.com, she’d have migrated over.
This is scifi right now, but it’s getting closer, as P2P isn’t very usable if you can’t sync your account across multiple devices/nodes.
Alternatively someone could write a script to migrate synapse’s db into dendrite’s schema (or dendrite’s kafka logs), but it’s be pretty fragile given how rapidly both synapse and dendrite evolve.
tl;dr: jump in with Synapse today - the water’s great (even if others have been scalded in the past). Migration to Dendrite or Conduit or whatever in future should be transparent, one way or another.