My Casual group is becoming competitive

Only thing I can recommend is to plan your games in advance.

Ask someone to bring a list of similar power level as yours and set a time and place, it has been my experience that a lot of players will tailor their lists if you ask nicely (just don't expect this power level to be the default).

Of course, if your meta is taking the competitive route, you should really consider returning the favor every once in a while. It gets old to always having to be the one making concessions for the sake of someone else's fun.

You could also try to organize a League or Campaign, those are usually laid back.

Adjust

both expectations and behaviors. I'm a die hard casual. But the guys wanted me to go to the Steam Roller. So I did.
I play out of theme weirdness but I also try to play with models they will be seeing in tournament lists. I started a thread on that line of reasoning specifically. But I fell off the meta bandwagon quite fast. It's just not my thing.

You have to understand that they will want to play more "meta" style list if you have them. So strike a balance. Play a meta list some weeks and weird stuff the next.

Try to really push for strategic plays some times, other times you can yolo and do something unreasonable.

But don't try to hold everyone to a single standard. That's a quick and easy way to alienate people. They will appreciate your attempt to play more the way they want so in turn they should play more lighthearted for you when things call for it.

Also; as said - organize a fluff campaign. Try to run caster draft or some of the holiday packets from last year. When you set the expectation that it's meant for fun. people (should) leave their try-hard hats at the door.

Thanks for the responses and advice. I think having two lists ready to play one of which is more competitive isn't asking too much as long as I can get games in with both lists.