Can I use MassTransit/Automatonymous saga without using entire MassTransit framework?
Automatonymous is (well, was, but still is) a standalone project. But it has seen limited adoption outside of MassTransit for the reasons you state: instance management, loading and saving instances, identity, etc.
Which is why with v8 of MassTransit, the Automatonymous codebase has been fully integrated to provide a seamless developer experience. Concerns such as instance/state management are handled by the saga repository, which is message-based. I haven't thought about providing any other affordances to dispatch events to state machines outside of the saga repository at this point.