I agree with SturdyErde that you should avoid having file shares on your DC, but I migrated my company off SBS08 a couple of years ago, and do understand that sometimes it is what it is. A high level description of my suggested upgrade path is:
Add the 2012 R2 server to the domain as a domain controller
Mirror your shares
Add your 2019 ...
This is a relatively old question but relatively in-vogue given the current work from home restrictions for many of us in regards to the pandemic.
A VPN-Less option is something called SMBStream which I'm assuming uses the QUIC protocol underneath the hood to achieve its magic. It accelerates access to SMB shares over the internet to almost LAN speeds.
while you can run file shares on whatever you want, why bother with such a large change? Both Dell to Pi, and Windows to Linux are relatively large jumps.
A rack server enables hardware features like 10 Gb or 25 Gb networking, expansion ports, dual power supplies, and a single vendor for parts. Pi is cheap and low power, yes, but only 1 Gb Ethernet, and how ...