Using child themes is a myth that you can live without. If you lived with modifying the parent theme all this time, there is no advantage in switching to use a child theme.
The implied promise of using a child theme is that you will be able to upgrade the parent theme without needing to redo your customizations. Unfortunately, most theme authors do not understand that this imposes a limitation on the amount of changes they should do in the parent theme when releasing a new version, and many times still release versions that break customization even when done in child theme, and this makes the whole concept break.
IMO a better alternative to child theme is to track the changes you have done in git, and when you need to do an upgrade of the base theme, you “just” merge your changes with the ones introduced in the new version.