Hi and welcome.
I also own an ETS320. Mine is currently dismantled while I do some modification work on it.
I wrote up a teardown of the unit on this forum. I will link to that later.
Ok here are my comments on improving the ETS320.
1. The fixed focus distance is a disaster and makes no sense at all. The ETS320 uses an Ex series camera chassis in a different case. The lens IS focussable using a simple tool that may be 3D printed. No disassembly is normally needed but there may be a rubber seal around the lens mount. This should not cause an issue though. The tool engages with little indents in the lens holder, just as on the E4 etc. There are two thin 'O' rings on the lens thread to set the close focus distance at the factory and secure the lens firmly in place. The lens may still be unscrewed further to provide closer focus. This gives the best performance. The focus may be returned to normal by just screwing the lens in until it presses gently against the O rings again. To focus the ETs320 for distance working, you need to completely unscrew the lens and remove the two O rings before refitting the lens.
2. The stand supplied is not great. I am attaching mu ETS320 head to a proper microscope stand with boom arm for larger PCB work and other tasks needing a longer reach boom.
3. A ZnSe Plano Convex or Meniscus lens may be used to provide close focus without adjusting the units lens but you are then limited to the FL of the chosen lens. cCommon FL's for laser engraver ZnSe lenses are 25mm, 50mm, 63mm and 100mm. There will be some distortion caused by the lens at it's periphery as Laser engraver lenses are cheap and cheerful affairs. A better quality ZnSe or Germanium lens will be very expensive and still limit you to one FL per lens. It is just like adding close up reading glasses to the ETS320

4. You cannot use ordinary glass lenses with Long Wave thermal cameras. You need to use Germanium, Special Chalcogenide IR glass or materials line ZnSe, ZnS or GaAs.
Regards
Fraser