Hi, I also had my brand new refurbished S9+ always rebooting after having successfully installed /e/ OS on it !
First you have to know that I’m French and new in the Android world (so it’s possible that I’m describing things a wrong way).
Here are a chronology of what I have done (to install /e/ OS and get my S9+ back after the bootloop issue) :
- First I got a refurbished S9+ (Model SM-G965F/DS) with Android 9 Pie (Original Samsung ROM, I think the update was 2019-08-07) preinstalled (and I have just powered it ON one time to verify that the hardware was OK using hardware test apps).
- I followed the TWRP method to flash the latest /e/ OS ROM (because I found it faster and in the past I successfully flashed /e/ OS on a Samsung Galaxy S7 following this method).
- So I first needed to install TWRP. But the phone was locked in “KG Prenormal State” and I was not able to enable “OEM Unlocking” to install TWRP via Odin. So I fixed it following the method shown in this video (this method worked to bypass the “7 waiting days” to get the OEM Unlocking option available in the Developper Settings)…
- Once TWRP was open, I followed the wiping instructions (Dalvik / ART Cache - Data - Cache - System) but I got errors messages… so I finally solve them by first formating DATA partition (which I think was encrypted by the default OS by samsung).
- Then I have transferred the /e/ ROM and tried to flash but I got one blocking error (“Error 7”) so I followed this post solution to modify the ROM to bypass the verification and be able to successfully flash the /e/ ROM.
- Then I got the same “always restarting / bootloop issue” flashing the device with this 3 builds : e-0.7-o-2019091423670-dev-star2lte / e-0.7-o-2019101026628-dev-star2lte / e-0.7-o-2019102428284-dev-star2lte.
- I have read the last reply from archie and tried to flash Stock ROM Android 8.0.0 via Odin but got errors (displayed on the smartphone in download mode) because the bootloader version was version 6 and the ROM bootloader version was 2.
- So if I have correctly understood : it’s not possible to downgrade to a Samsung Stock ROM with older bootloader version ;
- And according to the firmwares list from Sammobile, and because my bootloader version is 6, the older Samsung Stock ROM I’m able to install is “2019-08-07 - PDA G965FXXU6CSG8” ;
- And the only way a downgrade is possible from Android 9 to Android 8 for the Samsung S9+ is if the smartphone have not been updated more than this version of Android 9 : “2019-03-26 - PDA G965FXXU2CSC8” (because the bootloader is version 2 and the lastest Android 8 Stock ROM is also version 2 “2018-12-03 - PDA G965FXXS2BRJ6”).
- So I have been forced to flash the stock ROM (update 2019-08-07) to be able to use my new smartphone ; The bad news for me is the android firewall app I’ve installed is now notifying me about so many outgoing connexions from the system and default apps, it’s crazy !
I’m disappointed because I buyed this device only to have /e/ OS on it and I don’t wanted to be forced to flash the default Samsung ROM 
I also read this thread “Roadmap for Android 9?” and I’m also wondering : when I will be able to flash /e/ OS based on Android 9 Pie on my S9+ bootloader 6 ? ; and before the /e/ OS team release a Android 9 ROM for the Samsung S9+ : can I update to bootloader 7 or not ?
I know /e/ OS is still in beta and a lot of work is needed to accomplish the compatibility with differents smartphones ; I just hope the waiting time will be short 
; but I am also confident about it now that I see Refurbished Samsung S9 and S9+ with /e/ OS preinstalled and provided by the /e/ foundation because I think that is it possible for the new refurbished S9 and S9+ got by the /e/ foundation team (in charge of the smartphones flashing process) to get the new bootloader preinstalled (which prevent the downgrade to Android 8 and the usage of the /e/ ROM based on Android 8) !
I hope my post will be helpful for /e/ OS users and developers (more specifically for the owners of Samsung Smartphones). Thank you to all of you being part of this community (you are wonderful) !