I then used the ubuntu system disk creator thingy program to write the.iso to the usb-stick. I had to activate legacy boot options and even then I only reached the Ubuntu-Boot-Progress-Bar where the system froze. Can't download ubuntu onto usb stick.

There should be option available to browse iso file from machine

There is already a 'Demo mode' option.

If you already pre-downloaded an iso file, Windows part of installation is finished in few seconds. Internet connection is also not needed for that part of installation. After booting into Ubuntu you see
There you can press ESC
If you select 'Demo mode' there is a live environment to browse the iso file.

Ubuntu 1704 Usb Bootable Iso Download Free

Note: The provided installer in live environment does not use Wubi configuration. If you decide to install it with Wubiuefi, reboot and do not select 'Demo mode' this time.

Besides, it is possible to browse file list at release site e.g.: ubuntu-16.04.3-desktop-amd64.list and ubuntu-16.04.3-desktop-amd64.manifest for ubuntu-16.04.3-desktop-amd64.iso

Additionally, Windows has the option to mount iso files as drive. see e.g. https://www.tenforums.com/tutorials/3579-mount-unmount-iso-img-file-windows-10-a.html

Free Iso To Usb Bootable

If you only want to speed up downloads with Wubiuefi, you can use parameter to disable bittorrent downloads. Bittorrent is the default value for Wubiuefi, but argument --nobittorrent disables this option.