S9+ randomly reboots in my pocket

I am sorry that my “low impact” suggestions have not worked and that you wasted time on my idea to wipe cache which we learn here How to wipe cache partition in /e/ Recovery? - #2 by aibd is not possible with /e/ Recovery.

There is a TWRP Recovery for star2lte, but these suggestions were only meant to be easy ones, I am not very confident that this will be a solution. Perhaps someone else can offer an idea.

It would be very much a last resort to reinstall /e/. If you felt it had come to that you might just try installing TWRP as your recovery; wipe cache; test your device with the current install + your new TWRP. If this showed no benefit, then go on to a reinstall of /e/.

With Linux the TWRP would be as the above page says with

heimdall flash --RECOVERY recoveryfilename.img

With Windows you would use Odin with guidance here [HOWTO] Install /e/ on a Samsung smartphone with Windows easily in #5.

This does seem a bit extreme. I hope you get a better suggestion.