Jerboa does this. I get content from all instances and I can block and subscribe as I want. I have no complaints at all as of yet. If you are using android, I highly suggest using jerboa
I'm using Jerboa now with multiple accounts. Currently you have to select which account you're actively using. Each account has separate feed/subscriptions, which I prefer.
I don't see an option for 1 combined feed with all accounts subscriptions. Would be nice to have as an option though.
Subscriptions should only be stored on your home instance (that's why Lemmy surfaces monthly active users instead of subscribers) and the instance owners could see it somewhere but they are not world visible, no. However, upvotes are world-visible as far as I know, it's just that the UI doesn't show them.