I’ve been attending Warmachine events in the UK for years and they’re usually run through https://www.tiebreak.co.uk/.  They’re more tournament /competitive than Stargrave, but it works well.  Might not fit what you need though.  If you’re only having a few people then you might be able to run in on a scrap of paper.  When I’ve hosted 6-8 man tournaments in my house we just run everything from a whiteboard.  I’ve heard American Warmachine players like https://www.bestcoastpairings.com/ but I’ve no experience of it.

As Sundancer said, get the money upfront, and make it clear there’s no refunds from the date you can’t get your money back.

If this is for new players, maybe offer some demo evenings in the run up to the event to teach the rules and build hype?

Have a plan for the day.  Have a timetable for rounds and a plan for how to handle anyone overrunning, otherwise you could find the final round ending 2-3 hours behind schedule.  Make sure everyone is aware of all of this at the start of the day.

Get help.  8 friends you can manage easily enough.  You all know each other.  No pressure.  8+ strangers, each with their own expectations, and possibly different rules understandings and different gaming etiquette could create issues and a demand on your time.  A second pair of hands, and a second brain for solving rules disputes can be a big help.  You’ll also need to pee sometime and someone should be available in the hall.

I’ve never gone to a narrative event so I’m following this thread to see what comes up.  I do have Stargrave on my To Do list.