By creating such a large custom field, and by allowing your client to save so much complicated data, you are asking WP and ACF to perform a lot of complicated logic that WP was never built to do.
Please take this into consideration when looking for a solution to your memory issue.
Perhaps you have other plugins which are using up memory?
Perhaps your theme contains some code which is using up memory?
I would increase your memory limit for now, and think about optimizing your code.
Perhaps such a nested repeater field would be better broken up into individual post objects. This will allow you to save smaller chunks of data.
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