Hi,
The tags and the categories are taxonomy terms both, so their relevance is calculated alltogether, it cannot be separated.
You can change the keyword relevance modifiers under the Relevane Options subpanel. I would suggest giving a small number for the post content, a higher one for the taxonomy terms: https://i.imgur.com/NDL4ebB.png
If you are planning to use the index table engine, then it will also take the keyword occurence into consideration, and these numbers can be set for higher values as well.