Thanks for confirmation, SM-T720. Agreed, I see the device delivered as Pie/10.
Sammobile lists Android 10 as the latest firmware for this device. It is probably required that the device is fully updated before start in order to receive /e/ R. (This can be a cause of Error 7.)
I agree on sequence, but you can double check, the order of play is expected to be as our instructions (in OP).
The thing I had been specifically addressing in this thread so far is KG Prenormal.
KG Prenormal would always explain multiple fails. Prerequisite to fix.
Can you tell if it is an issue for you? (Can be read from "Download mode screen).
Edit, re Heimadll frontend, a self-biuld is ideal to be up to date for doing R, great. 
. Seeing your
sudo ./Heimdall/build/bin/heimdall flash
I would be interested to know if you find in Heimdall/build/bin the command heimdall-flash-frontend, it seems to show in his repo. The possible advantage is that the GUI might reveal blockers that would only show with obscure command line options, possibly!
Further Edit, thanks to Installation /e/ sur Galaxy Tab S5e - #38 by piero in this xda thread https://forum.xda-developers.com/t/rom-official-gts4lvwifi-11-lineageos-18-1.4169395/#post-83600521 @LuK1337 provides – bit more friendly stock → lineage instructions – which includes a couple of wise steps, (only after KG Prenormal is lifted.)
I see I accidentally introduced a misspresentation about libusb errors in Heimdall and Error 7 on the device.
In any case of repeated Samsung fails, it is always worth trying another cable!