sp_Blitz taking too long to complete
This can happen when the server is under heavy load, or some part of Blitz is getting blocked.

Rather than jump to Activity Monitor, run sp_BlitzWho, which you should have also gotten with the First Responder Kit. Go ahead and update your question with the exact command you ran "and also selected the common settings in the script at the bottom" doesn't tell me exactly what you ran. - Erik Darling

Try using @CheckUserDatabaseObjects = 0 to skip what's inside the user databases altogether. Then, when it's taking over, say, 90 seconds, run sp_BlitzWho in another window to see what query is currently running. Then you can copy/paste that into your question to help track it down.

Also, try sp_BlitzFirst SinceStartup = 1, and copy/paste the top result sets into a spreadsheet (or take a screenshot) - that'll help us use the top wait stats to understand what's happening on the server. - Brent Ozar