I need to click on a tag, let say "#Essential", and review only cards due today with that tag, by schedule, not cram them all. Custom views don't seem to reduce the number of cards for schedule review.

This functionality already exists and works for Decks. When I click on a deck, I only get cards I need to review today in that deck. I use it to break the cards and prioritise by review time by topic. But sometimes, even in a topic, I need to make sure I reviewed the Essential cards, or only cards with practical exercises etc.

Is that already possible and I just cannot figure out how to set it up? Or is this a feature request?

Unfortunately that's not possible at the moment. It would have to be a feature we add in.