I've been there. It can absolutely be miserable. With groups like this, you have to focus on small obtainable goals and make it a big deal when they accomplish them. Find one thing that you can make sound good this week(concert f, a whole note that starts and ends together, Bb scale, clean 8th note articulations, something...). When they accomplish it, make a really really big deal about how good it was and then never waver on your expectations if they try to go back to old habits. ("Remember when we did this yesterday, we started the note together and it sounded great? What happened? Let's make sure we do it again today so we continue to sound amazing!" or something along those lines)
If you don't use a method book, you should try to get one. I usually have success using a beginner book that they didn't use their 6th grade year. They won't really know it's a beginner book. Insist that they start together with their horns up and end together. Beginner books help fill in the gaps of kids that may have forgotten or need extra help understand some basic concepts or notes. At the very least, it'll help them establish habits of not talking or goofing off because they don't understand something. Make a game of concert F around the room of which section sounded better in tune or balanced (whatever you're going for that week), or a line in the book. Do chair tests. Play measure by measure if you have to, short snippets of playing followed by short snippets of instruction if needed.
Bottom line, it's exhausting having to keep your thumb on them like that, but it will help establish good rehearsal habits in the long run and they'll feel like they're accomplishing something if they have small, attainable goals set by you. Find something once and week and make it happen.
My group is big, but is mostly populated by kids who are middling on their instruments. I'm miserable. It takes us forever to get anything done because they never shut up. I've been told by a nearby director to "just have fun and make it fun for the kids." How do I do this when we struggle to be mediocre? There's no drive or desire to be anything more than seeing decent off in the distance. How do I make something fun when it all sounds so terrible?