I may be picky, but I wouldn't leave at 7am (we love a nice breakfast) in order to make a flight that still has a finite risk of cancellation. We drove to Rossaveal and it may not take longer if you're next stop is Clifden after your return, since both drives are on the way to Clifden. The marina and the ferry are only a stone's throw apart (slight exageration). Another point in the ferry's favor is that the Inishmore airport is a 45 minute walk to Kilronan, so you'll need a taxi or shuttle, if there is one. It would be an interesting challenge to see which method was actually faster. When your taxi gets you to Kilronan, the horse traps and tour vans you might use to get to the countryside might be gone, since they swarm for the ferry to unload. Maybe there's a similar frenzy at the airport that I'm unaware of. You'll be able to hire a bike or e-bike in Kilronan no matter when/how you get there.
I'm a big fan of flying when it makes solid sense. The risks of cancellation are dependent on when you're visiting. If the experience of a tiny plane is on your bucket list, have at it.
I'd strongly recommend spending the night on the island, as the character completely changes once the hoards of tourists leave on the last ferry. Much better experience. Music at T. Joe Watty’s, depending on the day.
As an anecdote about the ferry choices, our host in Kilronan (Cla Ban B&B) refused to book a room if we said we were coming from Doolin, since cancellation was so much more likely - maybe her son worked for the Rossaveal ferry :-)
Our favorite B&B in Galway is permanently closed, so last time we stayed at 7 Boutique which had just been remodeled and was very nice and under introductory rates. It has since gotten spendier. It is right in the heart of the pedestrian area and parking was half a block away. We had friends that spent the night on an old fishing boat in the harbor that had been converted to an Air B&B or VRBO - very fun.
Upstairs at The Crane Bar is our favorite music venue.