I think all the replies cover the insulation issue. For the m2 your floor would have more benefit which sounds like it has.
I would build a little vestibule with stud walls and insulate them to keep the draughts out and replace the window.
As said above the bitumen under the highly likely asbestos containing tiles acts as your DPM.
buy a decent underlay that’s thick enough to hide any unevenness and put the flooring over.
Can you not just use a high closer paint such as miranol?
I have worked with epoxy but I feel you would need to create a form for it to set around the door?