Thanks many for providing feedback and inputs in this thread. One suggestion I would have is I think the title of this thread should be changed to 6.1.x/6.2 and maybe started separate thread on DSM 6.2.3-x as I think the initial post and earlier portion of this thread is no longer relevant with the latest release of DSM. Maybe it's just limited my personal experience.
I've spent past couple of days combing through this thread and various other threads on this forum for my 6.1.7-x upgrade to the latest version 6.2.3-25423. Brief background, I've started xpenology out with DSM 6.1 running bare metal with Jun's Loader v1.02b, migrated it to another bare metal machine, and then to a VM hosted on ESXi 6.7U3 and eventually settled with VM hosting on Proxmox 6.1 now. I've been running DSM DS3615sx 6.1.x stably for at least 1+ year so now I've decided it's time to venture into upgrading it to the latest 6.2.3-25423 version.
After dozen of attempts with new test VM and config changes, it worked out better than expected in the end for me.
What previously VM settings such as BIOS and Machine type worked for me no longer works with the latest version. Similarly to what @REK1 mentioned above, my test VM kept getting stuck on 56% with error "Failed to install the file. the file is probably corrupted (13)", changing vid/pid did not help. Instead, what's ultimately resolving it for me is having the correct args: -device ... information, most importantly, "piix3-usb-uhci" needed to be changed to "ich9-usb-ehci1". Alternatively, you can also do an qm importdisk of the synoboot.img and assign it to SATA0. That'd been more straightforward and worked for me as well, but personally, I'd prefer not seeing the loader visible as a mountable drive inside the DSM OS so I took the former route of mounting it as a usb device like what you'd normally do in a typical bare metal scenario.
For those who's using EFI instead of Seabios, latest version 6.2.3-25423 will NOT WORK. Also, if your VM is set to "Default (i440fx)", it needs to be changed to "q35" instead.
I highly recommends doing a MIGRATION to the latest update instead of in-place upgrade. This means creating a new VM, test your Jun's Loader v1.03b settings first and VM settings first to make sure it's working properly, then attach your existing data disks to the new VM and do the standard DSM migration as if you're moving your hard drives to another NAS device.
I'm on latest Proxmox 6.1-8. Note latest version works fine with Intel E1000 out-of-the-box. I did not have to use custom E1000e adapter. Here's my working VM config:
args: -device ich9-usb-ehci1,id=usb-ctl-synoboot,addr=0x18 -drive id=usb-drv-synoboot,file=/var/lib/vz/images/103/synoboot103b.img,if=none,format=raw -device usb-storage,id=usb-stor-synoboot,bootindex=1,removable=off,drive=usb-drv-synoboot -device ahci,id=ahci1,multifunction=on,bus=pci.0,addr=0x8
boot: c
cores: 2
hostpci0: 02:00.0,rombar=0
ide2: none,media=cdrom
machine: q35
memory: 4096
name: nas-prod
net0: e1000=AE:75:AE:DD:9E:B2,bridge=vmbr0,firewall=1
numa: 0
ostype: l26
serial0: socket
smbios1: uuid=92d896c6-9071-43ae-94d7-66df8e6e33b7
sockets: 1
vmgenid: 3d8f4923-4400-444e-b627-ca5e0e8e39b7
Post-migration, I did notice Jun's Loader keeps showing as mounted USB drive inside DSM admin, what fixed that for me was using the "FixSynoboot.sh" posted by @flyride in the other thread below. Now, I no longer see the loader mounted each time I'm login to the DSM console, but only see my passthrough disks, just like before when I was on 6.1.7-x version.
https://xpenology.com/forum/topic/28183-running-623-on-esxi-synoboot-is-broken-fix-available