Every show and every theatre have different needs - you have to adapt to the situation that you're in. It may be that there are some places where you don't need to do as much during the run because of support from the other personnel at the theatre.
It's important to keep up the blocking regardless of understudies- it's often the only record of it and can be used to "archive" the performance for the theatre. (Especially if there is no archival recording made). It would also be crucial if an actor had to drop out of the show and an emergency replacement needed to be trained.
Even if a director or AD present to give notes, an SM should still keep an eye out during the show for things that are out of whack. (An actor not in their light, for example). I always check in with the actors before they leave- it gives me a chance to remind them of the next day's schedule, give them notes (if there are any), and gives them the chance to powwow with me about how things went. (A scene that had a different vibe, a broken prop, a torn costume piece, etc)
In terms of a long show with few cues, there are some suggestions on
this thread. If there's not a lot of cues, there's not a lot of cues.

I'm not sure why actors would get performance reports? Performance reports are usually for the theatre staff/designers, and sometimes contain information that actors don't need to see. (A note about another actor being late, for example). If I have feedback from the show for the actors, I will give it to them in person after the show.
On my current show, since there is a union crew, I feel like I don't have a ton to do, pre- and post-show. A typical day is arriving at the theatre, checking my email, checking in with the HM about the house count. Saying hello/checking in with the crew. Double-checking the props and costumes once they are set. Blackout check. Turning the house over the the HM. Calling 1/2 hour, checking that all actors are here, and other calls. Calling the show. Checking in with actors and crew post-show for notes. Typing my report and sending it. That's about it- it doesn't feel like a lot. (Mind you, we have 2- and 3-show days, so often this process is repeated...) But I make it my priority to do all of this as consistently as possible. More of my job is about double-checking that everything is getting done on time and correctly, and liaising with other staff members.
And, sometimes there's not that much to do.