Mount a king thinly padded headboard to wall three

Buy two regular twin mattresses/bed frames, shove tightly together add a foam strip "filler strip" and use KING bedding on the bed. You will have ALMOST 2 feet at bed end and you won't know the difference when you're sleeping on it. Neither will anyone else.

Your bedroom is 8’ by 15.6’.
A California King mattress is 6’ by 7’. An Eastern King is 6’3” by 6.5’. A Queen mattress is 5’ by 6.5’.
None of these beds will fit against an 8’ wall and allow walking space on both sides. All of these beds will fit against a 15.6’ wall and will have walking space on both sides and will accommodate two night stands. An Eastern King and a Queen mattress will allow 1.5’ of walking space at the foot of the bed, just barely enough for an average person to sidle past the bed.
A Full size mattress is 4.5’ by 6.25’. It will fit against an 8’ wall with one normal nightstand or two small nightstands and just enough space to walk on both sides. It will fit on the 15.6’ wall with room for two normal nightstands and will have a bit more walking room at the foot of the bed.
A headboard which stands on the floor will add at least 3” to the length of all of these beds. An upholstered foam “headboard” mounted to the wall above the mattress will reduce the usable length of the mattress by at least 2”.
A standard metal bed frame is just a bit larger than each mattress size.
Your bed choice for this room is either a Full, a Queen or an Eastern King placed against the 15.6’ wall.

There is no point to a full sized bed for anyone but a kid, unless you always sleep ALONE . At 54 inches in width, two are less than crib width.

A WALL MOUNT king headboard mounted to the wall can be mounted ABOVE the top of mattress....allowing the mattress a shove tight to the wall.....thus using up 75 inches only in length

A queen is still 80 inches long, so no point to that either

Meaning the best solution is the one I gave you. Room to get beyond bed, and room for two to comfortably sleep. Very Comfortably. In fact, it is done all the time for two adults who prefer different firmness in mattresses..