The link I posted earlier is outdated now that you can just download TWRP from the official website.
Luke, is your Tab S5e kernel .config available on-line?
I'm trying to bring up TWRP on the brand new Tab S6 (gts6lwifi), but my custom kernel won't boot the machine. I wanted to compare my kernel config with yours to see if yields any clues.
I based my config on the one I made last year for the Tab S4. That kernel booted on the first attempt, but the Tab S6 is proving much more difficult. This is the first 4.14 Qualcomm device I've built for, so maybe I'm missing something. Or maybe there's something wrong with Samsung's kernel source. It needed a lot of fixes before it would even compile, but that's always the case.
Anyway, if you could share your Tab S5e config with me, that would be convenient for the purpose of comparison.