Use Later.com for Instagram. It’s free for basic features like scheduling posts. If you want to schedule stories, that’s a premium feature you have to upgrade to.
Later allows you to prepare everything for your post such as description, hashtags, location, etc.
I don’t remember if you can schedule carousel posts on the basic plan, but you can at least do single photos or videos.
Edit: As to Twitter, you may be able to share your Instagram posts to twitter unless you want them separate for reasons of separate metrics tracking.