If I wish to delete a user ID completely, please could you advise on the steps to take and re-allocation of the tasks she has completed/ content uploaded, without losing any functionality of that content.
Is deletion the best option, or is it better to restrict access through downgrading their role/capabilities?
It’s for your choice.
WordPress offers automatically move all content from user you try to delete to other author as an option. If you select it, WordPress will change author for posts, pages and uploaded media from deleted user to a selected one.
If you need to store authors history you can just leave user in the list with no roles assigned.