sbv3000

you should be able to login to the console as 'root' and 'your admin pw' to check the files and or if you have telnet enabled use winscp to check directories, assuming the box has an ip address (pingable?)
if you have a spare hdd then disconnect your raid, boot with that connected and install dsm with same station name/admin pw but no volume, on reboot check for same errors (dodgy usb, try a new one?) but if the clean dsm works ok then leave that hdd attached to channel 1, reconnect other drives and boot, the 'good' system partition on channel 1 should repair the array disks you can then disconnect spare hdd etc. maybe look at updating to 6.1/6.2 as well

Check the tutorial for setting up an xpe boot drive. you can edit the s/n in the grub file as you would the vid/pid. you should be able to edit the grub file on the existing boot drive and on the next reboot it will load with the new s/n. I've run a few xpe boxes on at the same time with the same s/n and never has a problem. however note that I always use 'made up' s/n's that bear no relation to a real syno one (except the string format) as I dont use any features that need a real s/n.

The testing I've done with 6.x and multipliers all work out the same, drive 1 seems ok, but any that are on other ports no show. I think its the same historical limitation of the 'syno bios'. Even my tests with the  multiplers using the chipsets used in genuine Syno boxes dont work. I think your best option is a known compatible add on card, simpler and more chance of support from the forum if you need it.

File Station > Tools > Mount Remote Folder
 
Make sure you have the ip address, folder path and user/password with rights on the remote folder

if you have a spare hdd and usb disk, try a clean install with the base 6.1 loader (disconnect your raid hdds), then go through the updates to 6.1.4 U5, see if the same problem arises. If it does not then use that 'new' loader disk with your raid disks. If it does then you will need a serial connection to diagnose

To get an idea of what might be a good cpu for your build, have a look at the real DS3615/7 and 916 specs and work from there. https://www.synology.com/en-uk/knowledgebase/DSM/tutorial/General/What_kind_of_CPU_does_my_NAS_have

there are a couple of 'Ryzen' threads from people who have installed, but have had issues and, either bare metal or ESX, check them before you upgrade as it might be better to go Intel for the same budget