I would tend to say
Sara (and all of us) have a lot to think about now.
…but it occurs to me that if I remove the parenthetical it’s clearly incorrect. Is it proper to say
Sara (and all of us) has a lot to think about now.
…that sounds really strange to my ear.
Also, does it change the rules in any way if I use an em dash, e.g.
Sara–and all of us–have a lot to think about now.
(I encouraged @Yoseph-Baskin to expand his comment into an answer as I believe it’s correct. Since he hasn’t, I’m going to make it one myself.)
“Though the parens should allow us to remove its contents to test what remains, this is different. The ‘all of us’ amends the singular subject by expanding it to plural. If you can see it that way, the subject changes to plural and takes on a plural verb: A single grape—make that two—are what I need right now.”