Thank you for the feedback! When developing @engrave, I'm trying to find the right balance between functionality and simplicity. The basic idea behind the Engrave is to help bloggers create their's websites and monetize it somehow so they don't need to do work other than writing. And our target audience is users that may have no technical skills at all.

Steem is just complicated and I want to onboard users so I decided to get rid of some functionalities like beneficiaries. As far as I know, it's useless or even misleading for most bloggers. But maybe I should create some sort of additional settings to enable advanced features. It could be disabled for default to not confuse newcomers.

I am on the assumption that all the themes currently available on Engrave are powered by the same code and just display differently on the screen by using different CSS.

Every Engrave theme is different (coded individually). This is because we need to have some flexibility here to introduce completely different templates in the future. There were plans to allow bloggers editing themes but it's not valid anymore since Steemconnect integration and Single Sign-On are introduced and it's just the security concern. But simple CSS customization could be possible.

I am optimistic that it can further offer more functionalities like this

I just need to have more development resources and things like widgets could be introduced. I would love to do that actually.

I think you cannot add a tag if it starts in a digit

You cannot add a tag that starts with a digit on any Steem frontend because it's not a valid tag at a blockchain level. There are some limitations and we can't do anything with that. Also, the 5 tags limit is here because of the Steem API bug. Adding more tags decreases your audience since your post will not be listed on those tags and there is no point to do that. If you're using more tags it's like you use none of them.