Seems mostly fine to me, I game all the time on Linux (Bazzite gang 🤘) with a 3900X + 7900GRE, haven’t had any significant issues aside from needing to make sure clock speeds were configured correctly on the GPU. Two ram sticks is the way to go with these systems as sometimes they don’t support 4 sticks at full speed.
You’re right that GPU passthrough is definitely more for tinkering or advanced users with very specific needs (usually professionals who need Windows/Nvidia and choose to run it in a VM rather than dual-boot), with a budget to match. For a gamer couple, having fully separate systems is going to be much less hassle and more resilient against failure.
The one thing I would recommend changing is the power supply, it’s unironically the most important component in the computer because if it fails it can kill everything else, and the System Power 10 is known enough for being low-quality that discussions of that come up in web searches. Poor quality power supplies can damage your hardware and otherwise cause weird, intermittent issues even if everything seems to work fine most of the time, and will fail and shut off the computer when a good power supply would have just kept on chugging. Seasonic and Corsair are considered the best brands and have 10 year warranties - they’re more expensive, but they’re worth it. You want 80+ Gold or better these days, this is a buy once, cry once component.
If you don’t have a UPS, I would also recommend getting one at some point, either one big shared unit (if they’ll be close together) or two individual units. Having backup power will allow you to shut down the computers gracefully during a power outage, and prevents the worst-case scenario where the power goes out while the computer is installing updates and it turns into a brick.