Custom fields import/export issues

I need to import my client list of nearly 13k entries. Everything is set up in Classic custom fields (tried the new custom fields first). I did one import of my clients’ names and email addresses. Then I’ve tried to export to update the batch, yet it doesn’t export the custom field codes that I’ve made. I made one client with all the custom fields populated, thinking maybe that will force the columns to be made. Only 3 columns end up exporting. Someone please tell me there’s a simple workaround to this issue. I need a lot more details for my clients other than name and email. I made custom fields specifically so it will be possible to export directly from my website to automate the invoicing process. Thanks in advance