Convert comma separated values to array or object on backend

Stuck on (basic?) problem.

First of all : I have this working as intending using a datastore component, but it is too slow on the client side.

Working method:
Table with checkboxes that push selected record ids to the datastore.
Separate custom select outside the multi record form repeater.
I post the datastore and custom select variable to the backend.
The repeat sets the variable in the right column for all ids in the record.
So basically passing an array and a separate variable

Even if I limit the datastore to only take in the record ids it is slow. Say if I push 500 rows to the datastore it takes a couple of seconds before the data is in place. Posting to the backend and processing the data is fast.

Using the checkboxes to push ids to an array is much faster, but I am struggling to post them properly to the backend repeater.
I dont see the point of using the multi record form here since i can probably just push all the ids that needs processing directly to the backend

Any guidance ?
The input is set to value : array.items
How can I convert this to an array or object on the backend to pass it to the repeat

Or give me a better method if any

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