Are there currently limits on the number of categories available in the Simple Start subscription? Also are tags still available as of today?

There's no set limit for product categories since you can always delete unused categories to make room for new ones, Kington.

When the screen locks up or becomes unresponsive, it could be due to an unstable internet connection. I also recommend refreshing your browser, as this can help by reloading the page.

Once you add a new product category on the Products and Services page, ensure to refresh the page for the new category to appear in the drop-down lists. Please see the attached screenshot for reference.
 

To view all the product categories you've created, click Manage Categories on the Products and Services page. Here's a sample for a visual guide.
 

As for Tags, please note that this feature is no longer available in QuickBooks Online. It has been transitioned to Custom Fields, which offer a flexible way to capture additional information on your transactions.

On top of that, you can check out the Usage Limits to see how many Custom Fields and Chart of Accounts you're able to create based on your subscription plan.
 

This thread will remain open if you have further questions. We're always here to help you.