Read this about leaving a family group. Leave Family Sharing - Apple Support
Take out personal subscriptions to any services you use before leaving the first group. With things such as Apple Music, there is immediate loss of things such as playlists. With photos, what happens is more unclear. Read the section "What happens when you downgrade or cancel iCloud+" in this support article Downgrade or cancel your iCloud+ plan - Apple Support If you do not upgrade first, to me it appears that you do not actually lose content but your ability to add anything is disabled until you drop below the free level again. It could also be that after a period of time Apple might then remove content. However, in your case since you plan on taking out higher subscription level you might as well do it before leaving the current family to ensure a smooth transition.
Something to remember about iCloud storage is it isn't really a proper backup. It is designed more as a syncing service. Personally I would make sure that if you really do have 200 GB of photos that you also buy a hard drive (or two, in case one fails) and store them on it too.