Avoiding 'Decorating Cards' appears to be your unique selling point, as it is the most prominent headline text on your website and mentioned twice in your text here.

The meaning of this is obvious to you, while being obscure for me. Perhaps have a link within your site to expand on the meaning of 'Decorating Cards'. ( I assume it is prettifying. Perhaps it is initial card creation, or bulk loading. Perhaps it is includes other steps. ) Describe how your software is superior to putting minimal effort into using default anki cards with text and images.

OP is not a native English speaker and thought "decorar", the Brazilian Portuguese verb for memorize (with a "mindless memorization" subtext) translated to "decorate".

Epsilon editor from https://www.lugaru.com/

Stable and fast with EMACS-style behaviour and key assignments.

Has C-like extension language named EEL that you could convert / re-write the vundo tree in.

I love that Epsilon is still live and kicking. I've only ever met one person who used it but he was truly at one with the editor and traversed a large complex codebase with ease.

I used to use ϵpsilon, but then 32 years ago I got access to GNU Emacs, and I didn't look back. Maybe it's gotten better since then.

Does it support syntax highlighting for modern languages? What about having multiple languages in one file (.tsx or .heex, for example)?

I recommend Brother Black & White lasers. (Brother seem to want the business and don't try to abuse their customers.)

Epsilon https://www.lugaru.com/

Comes sufficiently close to be worth testing the free trial.

I don't know the file size limit, but it handles multi-GB files.

Does not fulfil the hex requirement. Recognizes and edits binary files but is not a hex editor. The previously suggested https://hexfiend.com/ looks better for that.

Unlimited line length, and can either horizontally scroll long lines or wrap them for display purposes.

Fast regex.

Unlimited undo/redo, including search/replace.