I had the same symptom as what you already described, regretfully and sorry but your phone is hard bricked, its not dead, just hard bricked. Sent for repair, it is cheaper than buying a new phone. trust me if you love your phone, it doesnt harm your money just for repair. But ask the repairer if it cheaper to just unbrick the phone because some repairer charge more for flashing the stock firmware after unbrick so basically
(jtag unbrick + flash stock firmware = $) then after the phone arrive and you can already access download mode then use the unbricking guide to restore firmware.