A proportional system will have multi-member districts. That’s the point. Unless you think that a national election can account for the needs and desires of a local population.

You don’t want to have someone from LA speaking for the needs of people in Kansas. Hell, you wouldn’t want people in LA speaking for the needs of people in Sacramento.

That’s where districts come in. To solve the issues with districts, you have two choices, either multi-member proportional districts, or shrink the districts down to the point where any resident can voice their opinion to their representative and expect a response.

Right now, the US has districts with more than 1 million residents. If even 1 in 1000 people have concerns that they voice, their representative will just ignore it all, because tens of thousands of voices are impossible to listen to.

That’s why smaller districts are key, even if you have multi-member proportional districts. No more than maybe 100k people per district.

A smaller district is also much harder to pack, crack, or otherwise gerrymander.

There is no downside, unless you have a favorite political party that only exists due to the current broken system.