You are correct Gmail no longer allows to select individual sounds for labels. If messages from a particular sender are way too important to miss, then create a filter for that sender using Gmail via the browser not the app. Select a message from the sender, then on the right, click on the 3-dots and select "Filter similar messages". At the bottom, select "Create filter with this search". Tick "Apply the label", then select "Choose Label" followed by "New label". Give you label a name and click OK. After creating the label, click on "Create Filter".
Open the Gmail app, go to Settings, select the account you have just created the filter and label from, if Notifications is not selected, then select it. Click on "Manage Labels", select the label you created for the sender, enable "Label notifications" and "Notify for every message".
Go back and Disable notifications for the other labels.
It is the only way for now to get notifications for that sender only until the ability to select ringtones for labels is restored.