Bricked a brand new OnePlus 6T with a botched firmware update! Am I screwed?

I have installed several custom Android ROMs in the past, but I'm by no means an expert, so please bear with me if I do something really stupid. I recently bought a new-old-stock OnePlus 6T with the intention of installing postmarketOS on it. I took it out of the box, set up Android, unlocked the bootloader, and so far all was going according to plan. postmarketOS installation with full-disk encryption seems to require TWRP, so I went to the TWRP download page. The latest version of TWRP would not boot with the version of the firmware currently on the device, instead simply showing the Fastboot logo indefinitely until force restarted. I downloaded the oldest version of TWRP available from the page, and that booted successfully, so I installed it to the recovery partition and immediately took a backup of everything TWRP would let me back up, then copied that backup onto my computer, just in case. I then trial-and-errored several more versions of TWRP to see the newest one that would work (3.4.2), and it did not appear to have the command the postmarketOS installation guide uses for partitioning (sgdisk) installed, so I assumed that in order to install it I would need to upgrade to a newer firmware so that I can boot a newer TWRP. I tried to download firmware updates via the system update in Android settings -- no luck, it says I'm up to date, but that can't be right! This is a six-year-old device; I guess the update servers are down. I then followed these two guides to download and flash a firmware update directly: https://xdaforums.com/t/guide-extracting-the-latest-oneplus-6t-firmwares-from-rom-update-package.4376047/ https://xdaforums.com/t/guide-flashing-the-latest-firmware-updates-to-your-oneplus-6t.4376055/ All seemed to go according to plan. I used AuroraDroid to download the app that downloads the firmware file, extracted the payload, rebooted into fastboot, flashed the various firmware parititions, and rebooted again. I had to delete "--slot=all" from all partitions except the modem for fastboot to do anything at all with them, and several partitions failed to write because they were "critical partitions" that the device refused to overwrite, but I thought nothing of it and rebooted anyway. I then got a big scary popup on the phone saying the device was corrupt and it refused to boot Android. Guess that firmware update was bad! I quickly booted TWRP from the recovery partition and restored the backup I took earlier. The restore got to 98% and failed (I don't remember the error message but it involved tar somehow), but I rebooted anyway. Now the phone will not boot anything other than fastboot! If I turn it on it goes directly into fastboot, and if I tell fastboot to "START" or "Reboot reovery", it just reboots into fastboot again. If I try to send it a firmware image via fastboot boot, it fails with Failed to load/authenticate boot image: Load Error. I do not want to send this factory-new device to E-waste only hours after I took it out of the shrink wrap! Can anyone help me? (P.S. I asked this exact question [on XDA as well](https://xdaforums.com/t/bricked-my-6t-with-a-botched-firmware-update-please-help.4704781/) in the hopes of a faster response.)