It’s done - /e/OS-T with e-Recovery is installed on my SM-T500 and runs at first sight. Hurray!
But it was a very difficult, hour-long birth, because I had to completely install Samsung Stock Android three times, including the bootloader unlocking process.
First I tried to install LineageOS 20.0 OFFICIAL twice, then /e/OS-T by @ronnz98 once. Three times the installations failed at a very early stage in the same way - i.e. already after flashing the vbmeta.
Samsung BIOS error message:
Only officially released bineries are allow to be flashed (vbmeta) + Security Error
I proceeded meticulously as described in the Lineage installation instructions. After flashing the vbmeta, I was done. I was unable to perform the second step, flashing the recovery, because the system kept getting stuck in the same place after a reboot (see screenshot 1).
The solution was finally to install an older vbmeta and TWRP recovery, which I had already used successfully to install a crDroid-GSI. Contrary to the lineage instructions, I installed both files at the same time: TWRP in the AB slot, vbmeta in the USERDATA slot of Odin (heimdall).
When the TWRP was running, I installed the e-Recovery T-V1.18 permanently and then the @ronnz98 ROM /e/OS-T V.18.
The next manual update will be very easy, because it can be installed without a PC, but with an SD card in the SM-T500 via adb sideload without data loss. Note: Not every device with e-Recovery offers this option.
I am looking forward to other user experiences.