^^^ In which case, I think that I've answered your question.
If you can get the P&O website to play ball, and actually allow you to make a booking, you need to ensure that you use the 'add-ons' option to book the bus service between the port and Rotterdam Central. Then it's an easy hop on the train.
However, irrespective of any sample dates that I entered on the website, it kept telling me that there were no bookings available. I hope that you'll have more luck than me:
https://www.poferries.com/en/routes/hull-to-rotterdam/farefinder#route
PS. A belated reply regarding your post about possibly moving to Liverpool:
In my opinion, it would very much depend upon which
part of Liverpool that you were thinking of moving to. As an analogy, if someone said to me that they were thinking of moving to Sheffield and that they were considering a house in somewhere like Page Hall or Fir Vale, I'd probably advise them to think again. However if they were considering Fulwood, Ranmoor, Totley or Loxley, I'd definitely suggest that they should go ahead.