I'm also catching up with this thread after reading your post on Amethyst Ice's and I'm so sorry your first post on the forum has focused so much on cage size etc. I had not seen your thread before or posted on it so didn't realise you were a newbie or were finding things a bit critical on here.
Everything you described at the start sounds great - lots of people use Carefresh. The Kallax conversion sounds great too. I think sometimes we can give advice when it isn't asked for
It sounds like you have researched it all well. The key word these days is enrichment, which is more important than how big the cage is (although most people would agree it needs to be a reasonable sized cage to fit the enrichment in) - and enrichment is basically variety - plenty of tunnels and hides, different levels, overhead cover - places to go, things to do.
You sometimes see people with huge cages with a lot of open space and not much in them! And that's not ideal for a hamster.
My personal thing is houses. I think they need a good sized house that's big enough to build a big cosy nest and even to fit a litter tray inside. They like a nice big dark house - a shoebox house is ideal.
Diet is always a tricky one in the US and I am no expert on US hamster mixes but I would agree that Oxbow isn't good for hamsters unfortunately.
I just wanted to add that I made many many mistakes with our first hamster - and it took a while to get things right. Whereas you have done your research and are getting things right already
I am very much looking forward to updates on the finished cage.
Do please keep updating us
. The one thing most people on here have is a passion for hamsters and their care and I am sure no criticism was intended - cage size is something that often crops up with various opinions and some strength of feeling sometimes.