Restricting Opportunity Access within shared Accounts

I belong to a Business Unit that needs to restrict User access to specific Opportunities within shared Accounts, e.g. Account X has many Opportunities and some of them should only be viewed or accessed by certain Users.  Is this possible and what would be the best/simplest way forward?  Could the sensitive Opportunities be tagged with a custom field and then Restriction Rules be established to limit which Users can view/edit said Opportunities?  If so, any advice on how a relatively naïve user could start this process (there are literally hundreds of Restriction Rule videos out there and advice on filtering them would be appreciated)?  If Restriction Rules is not the way to go, any advice on how to proceed?

Thanks in advance!