As a Kentish man (or Man of Kent, I can never remember which), I’m not sure there are any venues of note in any of these places. Rochester has The Royal Function Rooms, which touring bands have played in the past, but it’s rare (I saw The Wedding Present there last year). There is Riverside Studio One in Chatham, but very little has been put on there. Also Sun Pier House.
Haven’t known of anything much in Canterbury since The Barn closed. Think the Penny Theatre is still a going thing, but not much of note being put on there. You might get the odd gig at The Gulbekian.
Maidstone has nothing other than pubs with live music licences , as far as I know. Certainly nothing worth recommending. Maybe the odd band playing Pizza Express, other than local covers bands.
There is a new place in Dover called The Booking Hall, which seems to be getting some interesting stuff. Folkestone has the Quartehouse and Leas Cliff Halls. Margate, Tom Thumb Theatre and the Winter Gardens.