Thank you @FediTips
Found out how easy it is to preview and schedule posts today.
Mastodon has a schedule api so you don't need to be online at the time, just a client that supports it:
https://fedi.tips/scheduling-posts-on-mastodon/
Tusky and Tuba can both do this.
To preview a post you can send a direct message to no one. Then if happy redraft it and make public:
https://fedi.tips/creating-draft-posts-on-mastodon/
Preview is useful to see exactly how media will look when posted.