Symbol synchronization with unknown symbol rate
The one idea I have is to first use a symbol rate estimator, but the problem is that there will be some error from the estimator, then the error gets used in the symbol synchronization. Ideally, there is some synchronization method that don't need to know the symbol rate ahead of time or one that is resistant to small errors in the symbol rate estimate.

I mean, you need some tolerance anyways. No two oscillators in this universe are exactly the same frequency. And oscillator price pretty drastically scales with required long-term accuracy... so, you must assume that your receiver doesn't have perfect symbol rate knowledge, anyway.
Symbol timing recovery methods are typically resistant to small rate errors – look at the early/late gate, especially: You'll just see that you're a bit early more often than late if you're overestimating your symbol rate.
You just need to continuously run that.
This really isn't much different to phase recovery methods, which typically can also correct small frequency errors.

How could a multi-symbol rate system use an early late gate?

Uff, in such a system you'll often use a bit more complex synchronization methods, which take information from preambles, and, just as mentioned above, will keep tracking the symbol rate.