I agree, don’t bother ascending Owl or Hu Tao unless you have excess mats to use.
We are about one week away from the end of the month. You could wait until then to see if you get gravemaker, then decide.
Elena is a better center than GM or Marj. So if your priorities are cups, it makes sense to ascend her over the other two as your center position is the most important one. Elena is a great center to have and can anchor a raid defense at higher cups. However, Elena is only “ok” on raid or titan offense.
Marj is better than Elena for almost everything else (just not center defense), and there would be no shame in ascending her over Elena. She’s a premier red and should eventually be fully ascended for anyone fortunate to have her.
GM is also very, very good, but since you dont have any other 5* cannons, I would ascend marj before GM.
Given how difficult it is to pull 5*'s, come July 1 you probably won’t have marj or GM. That will make it easy for you. Elena