Nothing stops you from keep using Windows 7 with a sealed computer. What seems to be the issue here are either the old machine giving up or format compatibility.
On the first one you can buy a second hand Thinkpad T440 for cheap, install Windows 7, no internet required, and manually move the files. Only thing I would add on top of that is disk encryption and that can be done using Veracrypt (TrueCrypt for old Windows 7).
Now if you wanted to use a modern operating system I would test the compatibility of LibreOffice (under a well documented Linux, like Mint, Fedora or Ubuntu), I use it and has full support, especially for older MS Office formats. Still, I would only test at first, because format rot is a thing. Like the case of the US Navy were they tried to move older CAD files to a newer version of AutoCad but slight variations put lines where they shouldn’t and you can tell the issue, likewise you may see some weird artifacts if you try to move vast amounts of old doc files and open them up with LibreOffice, or any other FOSS software like Only office.
A third option is to run windows 7 in a VM, since it’s only a typing box anyway, you might as well spin up QEMU or an equivalent from the OS and hardware of your choice, just allow one USB for file transfer and cut off the internet.