@jteleia - I wish I could offer to help w/ the 3D CT image situation, but I’m not much good on computers (except for writing posts 
). I hope @KoolDude is able to continue helping you to a good conclusion w/ this.
I do want to say that I’m very sorry for how drawn out & frustrating your diagnosis & treatment have become, but I remain hopeful that you will get the help you need. I am praying for that. 
@jteleia I am really at loss why you can’t render 3D in 3D slicer following the brief tutorial. It could be that you are loading multiple series (I see multiple series loaded based on the dropdown box you have up there). When I am rendering 3D, I usually load one series and render one at a time. I never preload the whole study as that takes up a chunk of memory in my computer slows things down. May be if you try to load only that one series and then try to render, you might see it there. Of course, if that series is not loaded, you won’t be able to see it in the dropdown box.
By the way, you can try dragging and dropping the series on the 3D panel to quickly render it in 3D. May be that might work for you. Click this link (User Interface — 3D Slicer documentation) to see how that is done quickly.
You can also read up this link that discusses issue similar to the one you are witnessing. You might find a help here
Again, I have no clue why you can’t get past the first stage but I did warn you that it might take long if you are trying to upload huge studies. I have been able to upload one neck CT axial series without issues. Here is mine as example (https://www.dicomlibrary.com/meddream/?study=1.3.6.1.4.1.44316.6.102.1.20220522234638483.87563281508021434336). I can share it with anyone who has internet and browser to view it, that is the convenience of it. So the easiest way to share is to zip and upload one series at a time. DO NOT ZIP the whole study as that will make a huge zip file and might take forever to upload. I only zipped one series (Axial Neck series) and I was able to upload it fairly easily. One way to create a separate zip file for only one series like Axial Neck, is to load the series in any dicom viewer and then export that series to a file as dicom and then zip it. Once zipped, it will be easy to upload it to the site.
Well, if you find RadiAnt easier and meets your 3D rendering goals, then I think there is no point in pursuing 3D slicer. 3D slicer is open source and free. I do prefer it over the other ones for those reasons.