(The following text was essentially copy & pasted from a suggestion sent to the developrs)
While having a desktop, multi-platform application to convert between 3D file formats can be more practical and safer than doing it on one of the many online websites providing that kind of service for free, you should at least allow conversion between contemporary formats!
I mean, you don't even support Collada DAE (a.k.a. ISO/PAS 17506), which is a 21st-century format already outdated/obsolete, and replaced with glTF/glb (a.k.a. ISO/IEC 12113:2022). But there are dozens more that you should support, from the old (but still being used) Autodesk's DXF, to Unity's package format, to Pixar's Universal Scene Description (USD), to... Blender!
I mean, the only two formats I still recognise are OBJ (from 1985, and still popular thanks to its many extensions — note that you don't list which ones are supported) and 3DS (developed in the 1990s). The others are mysterious to me, and I don't think I've ever encountered them before.
While Spin 3D was probably quite useful in 2000, it's hopelessly out of date today — even Autodesk's own free FBX Converter (from 2013!) is more useful (even though there are 'only' Windows and macOS versions; but Linux fans already have a plethora of command-line utilities to convert between virtually any possible file format).
Spin 3D really needs a big update, if you expect anyone to continue to pay for it. In fact, initially I thought this was freeware (considering that there is freeware — such as, say, MeshLab — which does a far better job in converting between contemporary formats, and are also free and open-source) and was shocked to learn that it wasn't. It is cheaply licensed, sure, but for its next-to-zero usefulness, I wonder why you still list it as part of your portfolio!
(I'm almost afraid to look through your other file format converters; I might be equally shocked; but, so far, I just needed to do a quick conversion from Unity Package Format to glTF and was surprised to see that it wasn't supported... neither was DAE... neither was any of the formats I had... it struggled even with opening some of my OBJ files, especially those with textures and PBR extensions...)