All fixed guys, I will rewrite what I did in order to potentially help anyone else reading this. I didn't need to use that winntsetup, I needed to redownload the .iso from TechBench (Rather than from Microsoft, which is what's recommended ironically enough). Then I remade the USB bootable with Usb7ice, using the
1904v3.iso - after this I opened the Windows 10 .iso file with 7-zip and moved all the files into a new folder named Windows 10 on the USB bootable, the only file I couldn't move was install.wim due to it's size and the format of the drive. For this, I used gimagex to split the .wim file into two .swm files, these go into the same folder the install.wim was in (something like Windows 10/sources).
After this, I had trouble booting. I would get white lines on the monitor. This was due to boot settings in the BIOS, I can't remember what I changed, but changing from CSM or UEFI or vice versa wasn't enough, I had to manually change something to legacy or UEFI or something (sorry I don't remember now). This fixed it though and booted normally afterwards. After booting into the USB drive, all I had to do was open the Windows 10 folder and run setup. No issues afterwards, except a relatively long (around almost 1hr) screen saying "Windows is configuring, don't turn off" upon restarting - everything is working just fine now. I can definitely say I'm already missing Windows 7 though.
Thank you so much SIW2 and Megahertz07, I have learned a lot in the last few days thanks to your help in this thread. I really hope this helps someone else in the future, thank you again for the kindness and patience you showed when helping me.