BRIDGE (IMMEDIATE SOLUTION BUT NOT LONG LASTING): The Post and core of the upper left molar and the bridge of missing teeth is a good option if you do not want to go under sinus lift surgery. However, the bone support of the upper left molar is not good, so the longevity of that bridge is questionable.
IMPLANT ( NOT IMMEDIATE SOLUTION BUT LONG LASTING) : Only an implant is not possible in your case because there is not enough bone. Sinus lift and bone grafting are musts in your case. However, that is a time-consuming process, and if the dentist does not get initial stability in the implant, you have to wait for a few months without teeth.
BEST OPTION: Go for a sinus lift, grafting, and implant procedure. Immediately after that, go for a post and core, and bridge so you can chew on that side immediately. Meanwhile, grafting will create enough bone for implant support. When you feel any issue with the bridge, remove that bride and give a crown on the implant.