I would agree that excess all cause mortality is the best way to measure the impact of this pandemic across different countries. Unfortunately, we’re probably not going to have reliable data on that for months, possibly even next year. In the meantime, the excess deaths data that we do have gives at least an indication of things.
However, you have to be really careful comparing z-scores between different datasets. They are standardised to each individual population so, just because there’s a bigger spike in one country than another it doesn’t necessarily follow that said country has been worse affected. It could just be that they’ve had more stable averages in the past.
It’s another example where trend is more important than absolute score.