I don’t know that there’s a way to increase that limit.
In an ideal world, you’d be able to add the reviews to your site from Google Sheets using the Zapier integration and then also delete older reviews using the API or an integration to stay under the limit. But as far as I can tell there’s no way to automate the deletion of Collection Items, so you’d have to delete them manually which isn’t ideal with the volume you’ve got.
I’m pasting a link to another thread that could help shed some light on what’s possible in terms of deleting items via the API.