Very recently I've had good experience with Mork Borg games. Currently doing a Pirate Borg game with 6 players on a weekly basis and we've been playing for a few months now with a pretty long, extended story. I hate calling it a campaign, because there was no real defined ending that has been primarily player driven, but it's getting to a point now where it's naturally wrapping up. But, this system has rules light enough to make combat fast, quick and pithy, and lots of opportunities for randomization of events, items, outcomes and whatnot to put in unexpected flair in the story. It's very easy to learn, and great for building very random stories. The random tables of things is by far my favorite and is the best inspiration as long as players are willing to use those things as inspiration for something cool.

As an example, in Pirate Borg we had a situation where a character had access to "Ash" a drug-like substance that can be traded for profit or there is the option of ingesting it and a random table of effects. The effect we had was very non-mechanical, where a character sees darkness, shadows and feels lifeless. The player took this and ran with it, having the character feel very depressed and not wanting to do anything, really leaned in to that aspect of feeling lifeless even though there was no specific mechanic other than what the table was illustrating. It was really awesome!

I find that Powered by the Apocalypse games tend to be pretty good with 6ish players, but you also have to have very invested players who are willing to drive and narrate story along and not get too bogged down in mechanics. We did a few YEARS of PbtA games with great success, and they were immensely rewarding experiences. We played The Between recently and they were like. HARD LIMIT OF 4. We played with 6 just fine and it was an AWESOME ride and such a great story! I think a lot of this is about how your group adapts to being good storytellers and a good audience for each other. Have the mechanics be something that works to seamlessly tell a story rather than get in the way. If things are getting bogged down, it might be too much.

A few things to caveat though about our experience thought which I think leads to our success.

• We meet more often but for less time. Rather than meet once a month for 4 or 5 hours, we meet every week for a two hour session. Two hours seems to be that sweet spot where we can get a lot done, explore lots of concepts, paths and ideas, feel the pressure of time to get things accomplished, but not be overwhelmed or bored. It's also better for our focus since it's MUCH easier to focus on something for 2 hours and we don't have to kill an entire day or half day on playing a game. Friday night RPG is the highlight of my week, both running it and playing in it.
• Players have a LOT of agency. I don't often tell players they can't do something, but negotiate with how something might happen. We have found that with players being able to present ideas and outcomes as part of the world, we get more elements of the story that become new bits that can be folded in. These are "gifts" players give us that we can then expand on to create a much more rich environment.

These things are completely system independent and really what drives why we can have a group of 7 of us effectively play games like this for years and years on end and keeps us coming back for more.