Following up on Dual Boot Failure. I bought a slightly newer ThinkPad at Repowered for about $150. Similar specs: 8GB RAM, 256GB SSD, Windows 10 (but Professional Edition this time). No problems with Windows Configuration. Then I went on to install Xubuntu alongside Windows in a dual-boot arrangement.

Unlike on the other system, GRUB2 was installed. However the system would not boot through GRUB. Instead it went directly into Xubuntu. Going into the UEFI settings I could adjust the boot order so it would boot into Windows instead, but still not going through GRUB. Looking at a GitHub article I left the system in the Windows booting mode. Then, in a Windows Administrator Command window, ran:

bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi

I went into the UEFI settings and to allow both legacy and UEFI booting. Secure boot was off. This worked and when next I rebooted I got the GRUB menu. All better now.