It should be about the racing, but unfortunately, as with most things today,the branding is more important than the quality of the product.
There aren't enough speedway fans left in the UK to make a dent in the capacity of a large stadium, so it needs to appeal to a wider audience.
I used to attend Cardiff every year, with family members who weren't particularly interested in speedway, but loved the big stadium experience and as you say, "the day part"
Manchester won't recreate that unfortunately. In Cardiff, speedway takes over the city centre in an almost party atmosphere. In Manchester city centre most people won't even be aware that anything is happening.
The proximity of the stadium to city centre won't help either. I've not been to the NSS, but google maps has it as between 2 and half and 3 miles, depending on which area of the centre, so the 20 minute walk or under 5 minutes in an Uber that's being stated on here sounds optimistic to me. It feels like a trek to the Etihad which is nearer.
I'm sure it will sell out though, and the racing will be great viewing, so I'm really happy to have it as a 2nd British GP. I just wish they could reverse the dates.
Pitching Cardiff against a full football league programme & the opening day of the Premier League season must affect attendance from the wider audience. I can only assume that it's down to stadium availability, otherwise, it's madness!