I'm no expert, but if I'm reading this correctly, according to:
https://www.asus.com/support/cpu_supportthe Crosshair VI Hero needs at least BIOS 7002 (or possibly 7704) for a 3800 CPU.
If I've got your motherboard right, the latest BIOS is version 8101:
https://rog.asus.com/motherboards/rog-crosshair/rog-crosshair-vi-hero-model/helpdesk_biosYou mention the first one you tried was 3501. Assuming I'm looking at the right motherboard's page, did you try the BIOS flashback method on 8101?
Looking at some of the BIOS version descriptions, I see one possible compatibility issue:
Version 6808
2019/03/15 10 MBytes
CROSSHAIR VI HERO BIOS 6808
Update AGESA 0070 for the upcoming processors and improve CPU compatibility.
ASUS strongly recommends installing AMD chipset driver 18.50.16 or later before updating BIOS.
So, I guess I'd recommend heading off to AMD and grabbing the latest chipset driver you can find for the board, installing them and then using BIOS Flashback to flash to 8101.
EDIT: some notes: I'd never use any kind of operating system based BIOS flashing utility. Always flash either directly from within the BIOS or with BIOS Flashback (and for the distance you need to flash, I'd guess Flashback is the most likely to succeed). When I flash, I always first set the BIOS settings back to factory default, do the flash, set them back to default (with suitable reboots in between) and THEN set my real settings. And, don't take my word for those BIOS pages. Make sure the BIOS pages are for you actual motherboard.