[Cthulhu_]

> Where‘s the struggle?

> anything that gets cached can only be written by one service each

How do you guarantee it's only written by one service each? Sounds like locking across network boundaries, which is not easy.

> The writing services emit cache invalidation messages via SNS that cache users must listen to via SQS

SNS and SQS are both nontrivial services (at least you don't have to build / maintain them I suppose) that require training to use effectively and avoid any possible footguns

I think you're underestimating the complexity in your own solution, and you're probably lucky that some of the harder problems have already been solved for you.