I'm developing the same thing but for myself exclusively. To me personally, the HMRC MTD is an abomination. Before MTD was introduced, I could do my own books and could submit the results directly to HMRC via their website. No third parties necessary. Now they are telling me that I have to pay up to some third party to achieve the same thing AND to send my data through them. So either pay up or agree to be spied on or both. Sorry, but that's unacceptable to me. Orvellian technocracy dictatorship right there. Hence, I'm building my own open-source desktop HMRC MTD app for self-employed. It's not production-ready yet but it is getting there. Everyone is welcome to join me in development here:
https://github.com/AtmanActive/MyOpenUKTaxApp