It's because of thoughts like this that the number of tags per item is limited.

Initially they were going to allow unlimited tags until someone pointed out that you could use tags on your items as a poor mans key/value store for free.

I'm reminded of people who use S3 as an eventually-consistent database. Encode your rows in CSV and use them as names of empty objects. Query using paginated LIST requests for a prefix of the columns.

AWS already offers a free key value store - SSM Parameter Store. It’s severely rate limited if you don’t pay for it - but it’s still cheap.