I still have some characters on Mankrik and was wondering if i need to use the free realm transfer to the mega server now or if it will always be available
There hasn’t been any deadline given, as far as I know. And it’s unlikely they will remove transfers leaving characters on the old, locked realms.
You can not transfer and play chicken with Blizz. Hopefully they will not just pull the plug and force transfer you to any realm they feel appropriate