Unless you’re running a small form factor case, I might recommend stepping up to a full sized ATX mobo if you can squeeze it into the budget. As long as your CPU is supported it’s hard to go wrong, but I generally prefer the extra headroom of full sized boards; mostly for situations where I might need to toss in an extra drive or something, plus they’re usually a bit less cramped which makes them easier to work on. (Personally, I’m not a huge fan of Gigabyte. I haven’t sworn them off, but I find their software to be kinda clunky and annoying. Plus, I have two different machines with Gigabyte bios’ that seem to think “wake on LAN” means wake up randomly in the middle of the night, just because).

In general, RimWorld is pretty CPU bound (especially if you’re running a moderate or heavy mod list (modders don’t always write the most optimized code, myself included)), so a 3060 is probably overkill (assuming you don’t play anything else that might need it). From a RimWorld performance standpoint you’d get much more bang for your buck by cheaping out on the GPU and putting the money towards your CPU (1.6 brought in a bunch of multi threading and performance improvements, but generally speaking, a higher CPU clock speed means the game takes less time to do the computational work per tick. For the). Alternatively, if we’re talking as cheaply as possible, you might be able to get away with no GPU and using the CPU embedded graphics, it won’t be driving any AAA titles on 4k displays at crazy high refresh rates, but for RimWorld and the other games you’ve mentioned it should be plenty. Plus, the GPU is technically something you could come back and add later.

Also, it might be worth keeping an eye on the buildapcsales subreddit (there’s technically a Lemmy comm, but I don’t think it’s official affiliated and it looks pretty dead), especially if you’re thinking new hardware. If you live near a Microcenter, they’re known for doing CPU + mobo deals that stack with other promos. I’ve also had good luck with Best Buy’s price match (they won’t match Microcenter’s bundle, but if it’s a single item on sale it’s usually fair game).

If it helps, this is probably the list I’d be working off of, if I were buying today: pcpartpicker. It totals up to $645, which keeps it under your budget, but gives headroom to find a cheap GPU on the second hand market or a monitor or two if needed. The 7900X should easily handle the game (and pretty much anything else you throw at it), at $300+ it’s not cheap, but (as long as it’s not thermal throttling) I’ll do more for your TPS than a similarly priced GPU (7600X could probably be subbed in as a more affordable option (saves you $150). The boost ceiling is slightly lower, but still gets you most of the same performance at a much lower power draw and price tag).

Funny enough, it’s not all that dissimilar from my current build, build primary as a video transcoding rig (also a CPU bound endeavor) to feed my Plex server (and handle late game RimWorld, but I will admit to having been too busy lately to put that to the test).