Is there a widely used symbol for satoshi yet?

No, and there will never be such a symbol. Satoshi is useful for developers because it’s the smallest unit and all amounts are an integer in terms of this. This protects developers from even considering using floating point numbers to represent amount (floating point should never be used to represent money). The alternative is to write an arbitrary precision decimal-based floating point library.

It’s too useless and worthless to be useful for the end user.