Hallelujah, I got it!
I did nothing differently, but eventually it worked...very weird, but at least I got it! The final iteration to get it working was this:
1. Turn off fast boot option in power menu
2. Boot into bootloader via power menu, then power + vol down
3. go into fastboot
4. from a cmd window, fastboot flash recovery recovery.img
5. fastboot erase cache
6. from the menu, boot into recovery
7. phone boots to the OS instead of recovery
8. boot back into the bootloader via power menu, power + vol down
9. TAMPERED shows up, boot into recovery and it works!
note that i never used adb reboot recovery or adb reboot bootloader, idk if that matters, but I did everything from the phone.
thanks for all your help!