WPAI has a feature that will do imports from a URL on a schedule. An xml file can be uploaded to a URL, or better yet, if the supplier provides a URL for the and XML feed of the updates then WPIA can be set up to run from that on either a fixed schedule or manually.
Something I have done in the past was to provide a location where a client can upload a new export/spreadsheet. Then I set up a cron that ran once an hour to see if the date of the uploaded file had changed it would set the flag in WPAI to do the import. Then there was a second cron running, also every hour, that would start the import if the flag was set.
No, I am not employed by WPAI, but when dealing with large complicated imports, especially when ACF is involved, it is the best tool for the job.
If you want I can look to see if I still have some code for doing the cron thing I described above.
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