What a coincidence! This announcement was made just 30 minutes after I made this post. It should allow for you to get a players inventory without reaching API limits quickly.
For now, I found this solution:
Rolimons has an API called https://api.rolimons.com/players/v1/playerassets/userid which returns a list of collectibles the user has. You can use this alongside https://api.rolimons.com/items/v2/itemdetails to get the rap, value, demand, etc.