Is there a chemical propellant combination with performance better than Methalox but worse than Hydrolox?
There are, but there are no good combinations in that range.
Wikipedia's table lists a few things between methane/LOX at
3615m/s (369s) and hydrogen/LOX at 4462m/s (455s).
Of some note are pentaborane (B5H9) with LOX at 3758 m/s (383s) and diborane (B2H6) with LOX at 4016 m/s (409s); both are somewhat toxic, and can ignite in the presence of air.
Most fuels that work with LOX work a little better with liquid fluorine, so the common hydrocarbons that underperform methane with LOX will fall between methane/LOX and hydrogen/LOX using LF2 as an oxidizer, but doing so is dangerously impractical.