Chris,
As a DIYer what I have done in this situation where the door liner is not up to the job of taking the hinge Screws is to
- leave the door liner in place
- take off the architrave on the hinge side
- you say that you might have a 50mm gap between the door liner and the bricks so it might look like the image from this thread:
https://www.diynot.com/diy/threads/best-way-to-fill-50-60mm-gap-around-internal-door-lining.455898/
- get a length of wood that is as high as the door (say 2m), thinner than the gap, and as wide as possible (you might be limited by the screws from the door liner into the brick).
- Slide this into the gap so that their is wood where the hinge screws will be, hold against the door liner (not the wall) and screw through the door liner into this wood.
- you will now have a thick section of wood where the hinge screws are, and have doubled the thickness for the original door liner minimizing any bend.
- whilst able to look through gap, if needed put another screw through door liner into a suitably strong looking brick
- fill any gaps with Foam (reduces some noise passing between rooms,
- replace archtrave on the hinge side
- (note that when putting in new door liners I have often (when there is space) put a double thickness of wood onto the door liner on the hinge side so that the hinge screws have a double thickness of wood to grip on to)
Will this work for you?
sfk