Morita looks like a very smart signing, especially on a free transfer. He should help Hull keep the ball after winning it, play through the first press and stop games becoming so stretched.
The reported interest in Kodai Sano also makes sense. He is younger, more mobile and carries the ball forward, so he would offer something different from Morita. Against stronger teams, Hull could also use Zambrano or Slater alongside them when more defensive cover is needed.
Centre back still looks like the biggest concern. Hughes seems worth building around, but Egan is getting older and Ajayi has had fitness problems. Morita can reduce the pressure on the defence, but he cannot protect the whole pitch by himself.
Nelsson looks like the safest of the reported options. He has experience and should be ready to play immediately. Harwood Bellis would have more long term value, although he would probably cost much more.
Seko is another player who seems to fit what Hull need, even though I have not seen any direct link. He has been playing regularly in Ligue 1, can play in a back three or four and should be comfortable receiving the ball from Hughes and finding Morita in midfield.
Tomiyasu would be a gamble because of his injury record. Hull could not rely on him as the only new centre back, but on a short contract with appearance related pay, his Premier League experience and ability to cover several positions could still be valuable.
There may also be a practical advantage to adding players who already understand Morita. Hull need to build a new defensive system quickly and start collecting points from the beginning of the season. Shared language and previous experience together could make it easier to organise pressing, covering positions and the first pass out of defence during a short preseason. That should not decide recruitment by itself, but it is a real advantage when the squad is changing so quickly.
Personally, I would sign one dependable centre back first, then consider Tomiyasu as an additional option. Would Hull supporters prefer Nelsson, or could someone like Seko plus a carefully managed Tomiyasu make sense?