Honestly Blender is going to be your best bet. It's got the largest array of third party plugins, it rivals Maya and 3DS Max. Just generally there's nothing even close to it. Have you looked at Wings 3D? That's the only one that I can even fathom would be of interest to you but it's old AF, and nowhere even in the galactic neighborhood of blender.
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I've used Blender for simple models before, I was just thinking something simpler and more specific to low-poly might exist. But also in the past, I've not really enjoyed Blender (some of that might be personal hang-ups or bad experience egsuch as non-manifold geometry from modifiers, I liked the workflow and node-iness of Maya many years ago even though I definitely don't want something so expensive or bloated).
Have you looked at Wings 3D?
I've heard of it. I thought GFS might've done a video on it, though I did find Bloom3D in the browser instead, which seems like it might be another option (but the camera controls seem to suck) for some types of design. Maybe there's other similar simple CAD-like options?
@ExtraMedicated (for the top section)
If you're looking for CAD-like, try out CAD Sketcher addon for Blender. A youtuber by the channel name "Maker Tales" did a couple of tutorials on it, and it aims to bring parametric modeling to Blender in the way that it operates in more traditional cad programs.
Also try Plasticity. It's "CAD For Artists", and he doesn't do that stupid software-as-a-rent model. You buy it once (for a reasonable cost) and it's yours forever. 30 day trial to see if you like it; and it's cross platform, though not open source.
Well, it depends on how simple it is. I am not sure about how an add-on experience will be, but I've also heard of Bforartists but haven't evaluated it.
I have seen Plasticity, but honestly I don't want to deal with trials or even accounts really. And I likely am not going to create anything that justifies using paid tools.
I can't be much help here, but do let us know what you come up with!
I might not do much of anything (particularly as the tools seem not as good as I'd like, and I have a history of lurking with programming/development)
Not sure if you're interested in the 2D polygon stuff I've mentioned, but here's images of what I've already done: 1, Nim-lang code, 2, text format. Actually those are a bit outdated as I've worked a bit more on it (not quite happy with it because strip vs fan over-complicates it+data storage), also a square font makes the grid look better.
As far as free software goes, Blender is probably the best thing you'll find (although I haven't really tried much else). You'd need to do some tutorials, but it can do pretty much anything you need.