Hi Jiri! Being an ESL instructor and the second language learner myself, I find your idea brilliant! IELTS is definitely a struggle for a lot of English as a Second Language learners. To be honest, quite a number of my friends who are native speakers find it really challenging! With the technological changes in terms of language learning, a lot of tools are available at present. For example, you can easily avoid and eliminate so many mistakes using https://www.grammarly.com!
However, I think your idea is more functional and helpful for specific audience of the language learners who need assistance in writing in the first place. I also really enjoyed the presentation of your venture: very visually appealing and clear in structure. Awesome work!