Building a powerbank into a record player
Yes, you can hardwire a power bank to a device. The risk you are running into is you will forget to turn off the switch to the power bank when you plug in the external power barrel jack into the record player. What I would suggest is that you wire the 5V output of the power bank to the same format DC barrel jack, so that you can simply plug it into the record player instead of hardwiring it.