instant-meshes VS meshlab

there’s a free Windows program called instant meshes that’ll do it.

It sounds like you want better automatic retopology methods. There's Instant Meshes, which is decent, and Retopoflow, which is very good, but not automatic and not free. There's nothing that's fully automatic that's completely satisfactory, though.

Maybe checkout Instant Meshes for retopo?

There are other useful things available on Linux too— I use Substance 3D Painter from Steam for texturing, Krita for concept art and retouching, and this neat little tool for retopology. Again, nothing you can't get on Windows too, but it's so you know what you count with. You get a stable and reliable OS, full desktop customization to make it fit your own workflow, and a cool community in exchange. Whether that much is worth the tradeoffs depends on you.

Exactly. Run it through https://github.com/wjakob/instant-meshes then extract a normal map.

To be quite frank, you haven't really given quite enough context at all, but going out on a limb here the most common culprit of 3d files being too big is that they have an unreasonable amount of polygons. you can fix this by using the remesh tool/modifier (which will likely require some smoothing in the sculpt tab afterward) or using the decimate modifier. additionally if you're willing to use a secondary program, you could try a re-topology program such as instant mesh which is free, small, and the most clean & efficient at what it does, but if you aren't going to be doing 3d work often, its not necessary. After your poly count is low enough, export it as an OBJ or FBX file, and you should be good to go.

I’d switch to zbrush for this and project this mesh onto an already topology-correct one. If you don’t want/can’t use zbrush, and you’re out of options you might as well just give instant-meshes a try as it’s a free tool. I had somewhat successful results https://github.com/wjakob/instant-meshes

Instant meshes (https://github.com/wjakob/instant-meshes) is a free software that is very good at remeshing while preserving details

MeshLab is free software that can work with point clouds. I've only used data in .ply format so I'm not sure what other formats are supported.

I used a program called Radiant to convert to STL and then did some small edits using Meshlab.

The best open source way I have found is using MeshLab

You could start by importing the point cloud model into a software with native support for the file format. Some open source examples are Meshroom (https://alicevision.org) or Meshlab (https://www.meshlab.net). There are other softwares from companies like Autodesk that might be more intuitive but you'd need licenses to use them. Rhino and Blender can handle the file formats but might struggle depending on the size and complexity.

Maybe MeshLab