It’s better to use a pricing plugin for that. We have this plugin: https://wordpress.org/plugins/soft79-wc-pricing-rules/ . We’re not actively developing this plugin anymore, but we still use it ourselves and works fine. Maybe there are better options available.
Or do you need this functionality with a coupon code? That’s possible with our plugin, but needs a custom PHP snippet that we can provide.
I added the feature to the core plugin, because I think it’s useful for other stores as well. I will send you an email with the development version to try it out. Please let me know if the functionality is ok or if you have any suggestions.