That should be possible. One question, do you delete items from the list? If not, then just add a filter query where ID gt 300.
If items can be deleted, you'll use a similar filter query, but you will need to calculate the number for the gt condition. To do that:
Add an integer variable and name it FilterValue
Add a Get items action to your list and sort it by ID asc and limit it to returning 300 items
Add a condition and use a length expression to check that 300 items were returned.
​​​​​​​If less than 300 items are returned, then no exclusion is required.
This isn't needed if you know your list already has more than 300 items.
Add a Set variable action and set the FilterValue to the value of the 300th item.
This expression should work: int(body('Get_items')?['value'][299]?['ID'])
You can then add another Get items action and filter it to where ID gt FilterValue
I have not tested this, but it should work.
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