It has always been a pain to switch to classic only to click that manual share button and jump back to lightning and continue doing the work.
I have been working with LEX for a while and the other option I used all along was this AppExchange product.
Looks like our problem is now solved starting Spring ’21 release.
Clicking on sharing, allows you to create a share record.
Experience Cloud sites and the Salesforce mobile app don’t support manual sharing in Lightning.