I personally don't have a dedicated toolbox USB anymore, since any repairs I do now are in my home and I can just set up what I need when I need, but when I did have one I used SARDU to handle setting it up to boot into a grub menu that I could use to launch multiple live boot environments.

It was real handy to have Hiren's Boot CD, a couple live boot antivirus tools, live boot gparted, a live boot lightweight linux like puppylinux, and a live boot of a more "standard" linux like Ubuntu all on the same USB. Just boot to it and select which one you need from the GRUB menu.

Besides live-boot utilities, I worked on Windows most of the time, so I'd have the sysinternals suite handy, a few ninite installers exes for quick batch installs of standard programs, and a kludged together set of portable apps.

There's a handful of "portable app" launchers for Windows, and each framework has it's own library of compatible apps. I mixed and matched from a few of them with some programs that had official portable installs. I think the frameworks I used were "PortableApps" and "Liberkey".

I had a portable hardened Firefox, a portable PDF reader (I think SumatraPDF), Notepad++ (has an official portable mode), 7-zip or Peazip (can't remember, one has official portable mode), Bleachbit with the extended application support configs, the LargeAddressAware patcher (to allow 32 bit programs to use more that 4GB of RAM), and I think Teracopy had a portable mode as well.

If I had to set one up again now I would probably see if there was a portable way to carry Powershell around as well. There's a ton of stuff on Windows that's just easier to config and troubleshoot through it. I also might include some Windows 10 and 11 debloater/privacy configuration tools too, but I'd need to do some research on which ones are actually useful/good.