You're correct. A soldered in (potentially low capacity) eMMC drive makes your wife's laptop all but impossible to update. I had an old Dell tablet computer running Windows 8 in only 32GB eMMC and it became impossible to perform Windows Updates as the drive filled up.closed off laptop being used by my wife, where I'm pretty sure the eMMC drive is soldered on, so it wouldn't be able to be removed.
For this reason, I don't buy laptops with eMMC but pay a bit more for a SATA drive (or more recently M.2 NVMe). I also check if the RAM is socketed so I can increase capacity at a later date if necessary.
If you can find a Linux distro that's acceptable to your wife (and she doesn't run any programs that don't work in Linux) this would be the safest option. As already explained, if you continue to use unpatched Windows 10 after October, it will gradually fall behind safety wise, unless you shell out $30 for a year's additional support, Cheaper than buying a new laptop when computer prices might be increasing where you live.
I've just retired a 10-year old Acer Aspire R3-131T laptop with a slow dual core Celeron, because it takes hours to run Windows Update each month. I've changed the old hard disk for a SATA SSD with DRAM, swapped the single 4GB DIMM to 8GB and installed a new battery, then I'll gift it to someone with no internet access.
I've tried a number of Linux builds and have never really got used to the interface, plus the fact I can't run my standard Adobe software. Mind you, on a laptop with eMMC, you've probably only got 4GB RAM and that rules out more sophisticated programs.
Eventually, you might have to buy a new laptop. Sorry.