instant-meshes VS fSpy

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

Use fSpy.

If you're looking to align a locked off camera and create 3D scene based on a completely locked off shot with no movement at all, check out Perspective Plotter for Blender, or the tool it's based on called fSpy.

https://fspy.io/ try using this!

Save yourself some time and try fspy. https://fspy.io/