I am pretty sure I don’t need a child theme as I plan to only use the customizer for any changes. But I’d like to know, if at any point in the future I find that I do need a child theme, can the child theme be added at a later date? Will it cause issues with the website to add a child theme once the site is built?
you can add a Child Theme at a later date.
There are a few Customizer things that get saved at the Theme level such as the Site logo and Additional CSS – so those would need updating after a Child Theme is added. But apart from that all should be fine.