"Tags (often called “keywords” in other contexts) allow for detailed characterization of an item. You can tag items based on their topics, methods, status, ratings, or even based on your own workflow (e.g., “to-read”). Items can have as many tags as you like, and you can filter your library (or a specific collection) to show items having a specific set of one or more tags. "
No, that's the way to do it. Why is that limit a problem? To be clear, the limit is only for colored tags — which can be assigned via the number keys, and which you need to be able to tell apart in the items list, so it doesn't make a lot of sense to have more than 9.
Upgrade Storage
https://www.zotero.org/support/collections_and_tags
"Tags (often called “keywords” in other contexts) allow for detailed characterization of an item. You can tag items based on their topics, methods, status, ratings, or even based on your own workflow (e.g., “to-read”). Items can have as many tags as you like, and you can filter your library (or a specific collection) to show items having a specific set of one or more tags. "
Is there any other way to sort out articles to read. For example, mark titles of read articles in bold font?