Alternative to #23502.
Currently, when estimating the size an input spending a Taproot output will have once signed, we always assume the key path will be used. Even if there are script paths. This can lead to pretty severe fee underestimation if the script path turns out to be used in the end. So instead assume the most expensive between all script paths and the key path will be used.
This is still not ideal, as there may be a huge gap between the size of a script path spend and a key path spend. Still, this is less bad than undershooting the fees.