OEM lock-in can be a problem with OEM systems like Dell, motherboards and power supplies can definitely be custom and not off-the-shelf replaceable. I think the general consensus is once an old BI OEM system dies, you can just buy another used OEM system that's a few generations newer. Since your 7th gen isn't very old yet, I would price both options before making a decision, it's entirely possible it will be cheaper to order another complete used OEM business unit and ebay the spare parts.
If you do get the 3400G working, and can post some details about # cameras, whether sub-streams was being utilized, and what the CPU usage and power consumption (from the wall) was during peak usage -- it might be good information in case others choose to go that route. My the time you have to add a discrete graphics card, you lose all hope of benefiting from the Ryzen architecture reduced power consumption.