A major new resource in this category is the Kanji Learner's Course Graded Reading Sets. It's designed for people using the Kodansha Kanji Learner's Course, but basically anyone can use it as long as they already know kana and have some way to learn kanji. The first volume is free and gets you through the first 100 kanji.
From the website:
The series ...
There are a handful of practice exams online.
You can find 8 online reading tests here:
Another one here:
And if you scroll to the bottom of the second link, you can download additional tests as a pdf:
Perhaps some of this may be relevant in relation to your request for suggestions for calculating CEFR difficulty.
Duolingo has created a CEFR checker. https://cefr.duolingo.com/
Rather than rely on "syllable count and uniqueness" that you note FK relies on, Duolingo in its checker is using multilingual word embeddings (MWEs) and corpus frequencies ...
Usually you learn any language by listening, speaking, reading, writing.
Also wanted to highlight that sanskrit is not a dead language. By dead language, I assume that it is nobody really uses it actively.
I don't know about latin and other languages, but sanskrit is very much alive. The patronage decreased, but it seems to be making a comeback again.
*** UPDATE ***
The Russian Readers app is on the app store - and we’re publishing new Russian audio stories every week.
These stories are written specifically for native-English speakers learning Russian. We focus on introducing common vocabulary, and building up from there. Each story is professionally narrated, and translated into English.
Our goal at ...