The simpler we can keep this pattern, the less likely we will be to need to keep tweaking it.
I know of no DML (non-readonly) statements that are allowed to begin with S or start with a paren.
The only non-DML (readonly) statement I know of beyond select and parenthesized selects is the obscure "VALUES" statement -- a degenerate form of select that we do not yet support and would not be difficult to add to this simple pattern. Even if we missed that case and mistook it for a DML statement -- as we have been missing unions up to now -- it would only result in a small performance glitch.