@david @bloor @gyptazy @grunfink unfortunately, you can't (easily) migrate media/old toots. There are some external tools that can help with it, but I've never tried them. You can move your followers/followings.
Mastodon isn't that heavy - but media storage can become huge. snac won't download and reprocess all the media files (so it's light) and can give a good experience. It's not a fully featured Mastodon experience but...hey, it's not Mastodon 😉
You can also consider other solutions like GoToSocial. It has a more complete Mastodon API implementation as it has no native web interface (you can use all the web ones like Phanpy, elk, nicolium, etc)