particularly for something that is supposed to be a 6 kyu

It's not me who chose 6 kyu, altough I wouldn't call this a 5 kyu as well.

across 27 attempts and six solutions I had not a single attempt succeed

This depends on the server load, received inputs, and Python version you're using. When the estimated completion time is 12 sec, even good solutions are not guaranteed to work all the time.

the kata seems to be too tight and difficult

I've changed the tests a little to be lighter (at least the completion time dropped for me from 12 to 11 sec).