SPO modern list column width

I haven't tested this myself yet, but maybe the new list view formatting will solve the problem. It is in the portal Message Center and referenced as Roadmap Item 32990:

"With this change, you’ll be able to improve the display of views in SharePoint Online lists with formatting. The view formatting text describes the elements that are displayed and their display style. The data in the list doesn't change. Anyone who can create and manage views in a list can access view formatting from the View options menu. We'll be gradually rolling this out to Targeted Release organizations in late August, and the rollout will be completed worldwide by the end of September."