I spoke to my son, who did both state grammars and private schools.
He said in the final weeks, or final month - he just did Atom tests and they gave him confidence.
He didn't have a tutor at the end, but he did have one early on but it stopped due to covid.
Also the nerves of the test meant that he felt he dropped some marks but we had spoke about that and that he was not to get discouraged.
He said he isn't sure he would want to have more than one system.
He also did some paper tests but said that would apply if you were likely to sit paper tests.
Something I remember - try answer the first questions correctly as they can be adaptive tests.
If you make a mistake early on it is hard to catch it up.
My son did very well for the private schools that all did ISEB type set ups.
He did also say there was some content we didn't cover but that may have been for the state grammars.
In addition to Atom he said playing vocab games helped - basically I would throw him a ball, say a word and he would say want it meant and vice versa. He said this helped.
He also found the interview process with secondary schools really good and welcoming.
But by the time he sat at privates he had sat the big grammar tests.
I think you only have one shot at ISEB don't you and they share results, so I am not sure when you sit it but some privates sat 3 months apart so you can see that someone could conceivably have more practice on Atom.
Also have a think whether the school will sit it all in one session, or will split it over 2 days.
My son felt the long state grammar tests helped with this - and when he sat at a private school some students complained that it was long and they were tired.
On the day write a note of encouragement or play a favourite song to the test and afterwards celebrate the effort - that is what helped my son.
Actually you can start doing that now.
Also your tutor should have a good idea as to strategy.
All I can say that it is massively stressful experience, and keeping it light and together will help your child. Best of luck!!